Your own custom kits from GarageBand '11 should appear in the User Channel Strips section, like this:

Try to enter "Sound Effects" into the search field, if you do not see them.
One problem you might encounter: When I tried to use my custom sampler instruments, I got an error message "Instrument not found". I fixed this by copying all custom instruments from the Instruments Library in my User library to the corresponding folders in the System library.
Better make a backup, before you try to move things around in the libraries.
Locate the following folder in your User Library in your Home folder:
~/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/Sound Effects/
~/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files/Sound Effects/
Then move the contents of these folders to the corresponding folders in the System Library on your harddisk:
/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments/Sound Effects/
/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files/Sound Effects/
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