Have you checked, if your samples are still installed in your user library in your Home Folder?
The instruments should be installed in the folder
~/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/Sound Effects/Categories/Sound Effects/
and the samples in the folder
~/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files/Sound Effects/Categories/Sound Effects/
When I upgraded, my custom intruments remainded in that folder and are still working. For your custom instruments to continue working, you need to keep the GarageBand '11 installation, because the new GarageBand 10.0 does not have a customizable sound effect instrument. My custom Blues Harp still plays, because GarageBand 10.0 is using the GB '11 Sound Effect instrument.
You user library may still be hidden, as is the default:
- Quit GarageBand .
- Select the "Home" folder icon (the little house) in the Finder's sidebar and press the key combination ⌘J to open the "view options". Enable "Show Library Folder".
Then open the Home folder and open the Library folder inside and navigate to
~/Library/Application Support/GarageBand/Instrument Library/Sampler
Léonie