I am new to Minecraft. My son (7) is playing it in the creative mode. Now he wants to use train craft to build and ride trains. I already downloaded it and the pack appears in the settings, but we don't find anything about it now in the game itself. How can we start building a train and the railroad?
First you should check if the mod is actually loaded into Minecraft.
In the main menu of Minecraft (after pressing play in the Launcher) there should be a Mods button. Click on that.
In the mod list that appears on screen is TrainCraft listed?
If it is there, have you tried installing a mod like Not Enough Items (and its dependent CodeChickenCore) into that pack, and using their search bar to find the items? It tends to be better than the normal Creative menu.