This is an SSP mod, therefore you will need to add the file in the download to your .minecraft file. Please do not get all nervous, because this is a really easy process, just follow the simple steps I have made:

PC:
- Download the Mod from the link above (middle of the page)
- Open the Run application and type %appdata% into it and click OK
- open up .minecraft
- open up .bin
- open minecraft.jar with WinRAR or 7zip
- add the aah class file you downloaded to this directory
- delete META-INF. If you can not find it, you probably have already deleted it from another mod you have downloaded before

Mac:
- Download the Mod from the link above (middle of the page)
- Open up Finder window
- Go to your user thing (I think it usually has a home icon), if you're not there already, and open up the Library folder
- open the Application Support folder
- open up .minecraft
- open up .bin
- rename minecraft.jar to minecraft.jar.zip (select OK if it asks you if you're sure you want to do the .zip thing), and unzip minecraft.jar.zip
- this should create a folder called minecraft.jar Open it up.
- add the aah class file you downloaded to this directory
- delete META-INF. If you can not find it, you probably have already deleted it from another mod you have downloaded before