How did you install your GarageBand version? To be able to do the update you probably need to delete the Preference file, and if that does not fix it, uninstall Garageband, the loops, the Instruments library. Then reinstall it. This solved it for some posters (at least for me).
Make sure you have a backup before you do any if the following:
If your Mac came with GarageBand preinstalled, you can download GargeBand again from the AppStore. To be able to do that, you need to completely uninstall it first:
- Quit GarageBand. Move the GarageBand application to the Trash.
- Move the AppleLoops folder: "/Library/Audio/Apple Loops/" and the AppleLoopsIndex "/Library/Audio/Apple Loops Index/" to the Trash.
- Move the Instruments Library and other support files to the Trash from:/ Library/Application Support/GarageBand/
You see the /Library folder, when you double click the icon of your Hard Drive on your Desktop or in the "Devices" section of the Finder sidebar.
Don't empty the Trash, until you are sure, that you can download the files again from the AppStore. Then sign into the AppStore and download GarageBand again.
Or if you installed garageBand originally from a retail iLife '11 DVD, reinstall it from your DVD, then upgrade to the latest version using Software Update.