How to use a Soundfont in Garageband 10
First, you'll need to follow my instructions for Installing Soundfonts (SF2) on Mac. Once your soundfont is installed, you can use it to play music in Garageband. Here's how to create a track with your new instrument.
Select the Track menu then New Track
When prompted, select Software Instrument and click Create
Double click on the icon of the instrument in it's track to open the controls
Click on the Show Inspector button
Click on the plug-ins section
Click on Instrument, AU Instruments, Apple, DLSMusicDevice, then Stereo
In the window that opens, select your SF2 file in the Sound Bank dropdown
Select the Window menu then Show Musical Typing to open the keyboard
You can also use the Audio FX option to add additional effects like compression.