This post was created in general for a fix for the modpack 'Dude where's my blocks?', but this can happen with other mods hence why it still applies to general discussion.
So this took me an hour or so to figure out how to fix, and only around 3 minutes to realize it isn't a problem with FML but rather a problem with the Curse's version of the minecraft launcher (that is the minecraft launcher clone that is launched when 'play' is pressed on any minecraft thingy). The thing is, weirdly enough for some reason the version curse opens of the minecraft launcher seems like it is unable to read any data in certain areas even if the curse launcher was the one that created that folder/file.
This fix is super simple, find the 'Launcher.jar' file in the folder which you've specified curse to install the game to then open it using 'Java(TM) Platform SE binary'. That's it.
By default the .jar is here; "C:\Users\(your computer name)\Documents\Curse\Minecraft"
Hope this helps, if it did/didn't then don't forget to say so in the poll/comments.
The bellow also works, but is only for if the above solution hasn't worked. Sometimes moving stuff about can help.
Before we start, you can download the forge installer and let it install into .minecraft as an extra precaution but it is not necessary.
So the fix is simple, have curse install the mod for you then create a new folder where you ACTUALLY want the modpack to go (for this we'll say the folder is called 'WMB' and is on the desktop), go to wherever you told curse to place its stuff(e.g. if you told it to place everything in .minecraft) and COPY all of that into the folder you created[e.g. the WMB folder from before] (the folders you're looking to copy are these four; 'versions', 'instances', 'install' and 'Downloads' and make sure to copy over the 'launcher.jar' and '.curseclient' too). Then simply open 'launcher.jar' with 'JAVA platform SE binary'(which is what is usually used to open the launcher, it's just that minecraft auto assigns a shortcut on your desktop to open it), create a new profile in the launcher (bottom left corner) and set this stuff up when creating the new profile;
Tick the 'game Directory:' box and change it from the normal (whatever)\.minecraft(which is default) to the place where you made your new folder. (e.g. changing it from "C:\Users\User\AppData\Roaming\.minecraft" to "C:\Users\User\Desktop\WMB")
Then go to the 'Use version:' drop down list and set it to Forge, depending on if you let curse direct install into .minecraft or you let the forge installer install itself there then it will show up, if it isn't there just look for the 'versions' folder in the new folder you made (e.g. in the folder we named WMB) and move(or copy) the file into the .minecraft's 'versions' folder. (Make sure to press 'Save Profile' as you might have to restart the launcher at that point)
Next: tick the 'JVM Arguments:' box and copy the string bellow into into the input (which is the white box which probably already has some writing in it, but we're just adding a few more lines to it so don't worry about removing it):
"-Xmx4096m -Xms256m -XX:PermSize=256m -Dminecraft.applet.TargetDirectory="(the directory of where you made the new folder e.g. C:\Users\User\Desktop\WMB)" -Dfml.ignorePatchDiscrepancies=true -Dfml.ignoreInvalidMinecraftCertificates=true"
(this just enables Java Virtual Machine Arguments, which can solve a few problems here and there like FPS issues and more.
This is absolutely required to get the game working.)
"in the folder which you've specified curse to install" looks like I got a little exited and forgot to add "the game". I'll fix that.
I was saying that if you installed minecraft onto your desktop, look for the "Curse" folder on your desktop (otherwise it's the "minecraft" folder), the game will be in there along with the .jar that you need to open with java.
The default location for minecraft to install is at "C:\Users\(your computer name)\Documents\Curse\Minecraft", if you've forgot where you installed it then you should be able to go to advanced options on the curse launcher and it should say somewhere where it was installed.
Ok, so I know where the Curse Minecraft folder is, and inside of it are 3 folders: Downloads, Install, and Instances. The only launcher.jar file I've managed to find is inside Install, but when I open that with Java Platform SE Binary, I get the vanilla minecraft launcher that's from my .minecraft file. Inside the instances folder are the modpacks I've downloaded through Curse, but I cannot find a launcher.jar inside any of those. Inside the modpack foolders are all the normal files and folders you'd find in a .minecraft folder, execpt the Minecraft launcher. Instead of that there are two json files, manifest.json and minecraftinstance.json, and an item called .curseclient .
The launcher.jar will open a minecraft launcher that looks like the original (well it IS the original launcher) but the data it reads is not the normal .minecraft's.
Opening the launcher inside of the curse directory, there should already be a install of 'minecraft' ready to play where any mods will simply start like normal if you where to mod minecraft directly. If it isn't starting with mods (i.e. using normal minecraft) then you might have to go into the profile properties and change its directory from '.minecraft' to 'curse/Minecraft', but that should be done automatically.
The launcher.jar should be directly in 'cuse/Minecraft' and not in any sub-folders of the 'Minecraft' folder, if it isn't there then all I can guess is that you haven't downloaded and installed Minecraft through curse - but I doubt that's the case - in which even, if you have downloaded/installed it and it isn't there then try copying the launcher.jar file directly from .minecraft instead then create a new profile and edit the profiles directory to be pointing at 'C:\Users\(your computer name)\Documents\Curse\Minecraft'.
I honestly recommend - though - that if you are wanting to mod minecraft then you should use magic launcher instead, it's more reliable and actually works. That and it isn't to hard to use and actually gives to a real time log file (that you can clear, if it's clogging up to much ram) to look at to see if mods are breaking at all or if the game's crashed/done loading (and the log has fancy colours, who doesn't like fancy colours?).