I don't use Amplitube with Garageband but I do have it installed with an older version of Logic Express.
I read the instructions paparaptor posted and they are straight forward so I'm not sure what issue step 6 would pose. You paste in the folder that you copied in step 2. Unless the instructions are for and older version of the MacOS and the library has changed.
The instructions have you using the paste function in the edit menu but normally with Mac programs you can right click (or command-click) and get a popup menu with the paste function, or command-V will paste if the GarageBand app allows it.
What is the issue? You did copy the folder in step 2, correct?
Eric