Finding the .sf2 in garageband
I have a very old version of garageband 2.0.2. So most solutions didn't work for me. In the end I fiddled around and managed to find the sf2 this way
First exit Garageband
To get SF2 files into garage band put the .sf2 file into mac hard drive/library/audio/sounds/banks
Open garageband. Go to a track, get to the track info by pressing the eye button.
Look at details. Look at generator,in pop menu select DSLMusicDevice,
On the right hand side of that there is a box saying manual. To the right of that there is a little pencil icon. Click that.
In the box that appears look at the Apple soundbank synthesiser box. Change that to the title of the sf2 file that you want (it should show up now)
Close all that down, no need to save.
Away you go