After upgrading to Lion and Moutain Lion all of our 3rd party real instruments have disappeared in Garageband. They worked perfectly fine before the upgrade. I have tried re-installing, etc. And nothing. They are located in the appropriate folders in library/applications support/Garageband/iinstrument library, etc.
The still work in Logic, but have disappeared in Garageband.
I did get them to show up by opening an old project file with a missing instrument in it and when I click on that track, boom, all of the missing instruments showed up. But I wonder what the command is to get GB to "rescan" for the real instruments.