As a work around: While iTunes refuses to show in the Media Browser, drag your songs from iTunes to the Desktop, and then from there to Garage Band.
After a new installation sometimes it is necessary to remove invalid preference files.
- Quit iTunes if it is open.
- Open your hidden User Library and open the Preferences subfolder
- move the File "com.apple.iTunes.plist" to the Desktop - in this file the path to your iTunes media is stored.
Log out and log on again, restart iTunes, then see if iTunes now appears in the Media Browser
@LeonieDF: What is "hidden user library"? Where can I find that?
I meant the your User Library; it depends on the operating system if it is hidden or not - since MacOS 10.7 Lion the User Library is hidden.
The "Preferences" subfolder you need to access is located in
Your Home Folder -> Library -> Preferences
If you are using MacOS Lion you access this folder using the Finder's Go menu:
Finder -> Go -> Go to Folder
Then hold down the "Option" key, until "Library" appears in the drop down menu, select "Library", then proceed as above.