Quick answer
How to Install Garageband Instruments & Samples
1. Instruments folder should be copied into (User) Library/Application Support/Garageband/Instrument Library/Sampler/Sampler Instruments
2. Samples folder should be copied into (User) Library/Application Support/Garageband/Instrument Library/Sampler/Sampler Files
3. Track Settings folder should be copied into (User) Library/Application Support/Garageband/Instrument Library/Track Settings/Software
Long explanation that might help
Why is Garageband Instrument Installation more complicated than Logic?
You would think that installation of samples and instruments into Garageband would be simpler than a Logic Installation. Not so. Obviously Apple have decided to make Garageband almost a simpler cut down version of Logic (You could argue that Logic is just a grown up complicated version of Garageband).
Logic uses the EXS virtual instrument sampler in the insert of an instrument channel strip. These instruments are stored as .exs files in the User/Library/Application Support/Logic/Sampler Instruments folder. Each of these then references a set of samples (.aif or .wav files - the actual sounds), which are in a folder that can be anywhere on your hard drive. If moved, Logic should be able to find them. The EXS instrument has a user interface which allows you to edit the instrument parameters and raw samples.
As well as the EXS instrument in the channel insert, Logic can have various other plugins as effects on the other inserts of the channel strip, and you can save any channel strip configuration (ie the instrument and the other plugins) as a Channel Strip Setting. This is a .cst file and is saved to the User/Library/Application Support/Logic/Channel Strip Settings folder
So What Happens with Garageband ?
Well, Garageband is simpler for the user who just wants to simply record and playback. But adding exs instruments is not so easy. Garageband does not include the EXS 24 sampler, but it can read the .exs instruments in a roundabout kind of way. It uses the same .cst channels strip settings as Logic (except that it calls them Track Settings). Instead of loading a .exs instrument file directly (which you cant because there is no EXS24), you use a .cst setting (which can be created in Logic or Keymap) that contains the EXS instrument, but not the other plugins that you would normally save with a Channel Strip Setting. This means that in Garageband you need to install the exs instruments plus the cst settings as well as the samples. (In Logic you would only need .cst files when additional plugins had been added). That is basically it: if that makes sense to you then now you know what is going on "under the hood" of garageband. If not, dont worry, just have fun making music with the samples!
whole Article here
http://www.petethomas.co.uk/garageband-instruments-samples.html
Hope this Helps
