How do I get sfz files to play in gargeband?

That will depend on the format of your sound fonts.

Perhaps HangTime's post with the links to the FAQ will help:

Re: How can you add different instruments to garage band

Regards

Léonie

Follow this tutorial, I think it might be what you are looking for:

http://blog.plasticmind.com/beginners/free-garageband-orchestra/

The Alchemy plugin from CamelAudio is no longer available. You can use Szforzando from Plogue. First install szforzando. Then start Garageband and the plugin will be available. (If you had GarageBand open when you installed szforzando, close it and re-open.) To open the plugin and use it on a track, follow these steps.

1.Select the track to use

2. Open the Smart Controls

3. Click the Inspector button (with an "i" on it).

4. Below "Keyboard Sensitivity," click Plugins. You may have scroll the window to see it, or expand the window.

5. Select Instrument/All Instruments/Plogue Art et Technologie/szforzando/stereo. The plugin will open.

6. Click the "File" window, which should say "empty." A menu pops up with three options: import, empty, reload. Select import.

7. Navigate in your file system to an sfz. file and open.

8. Now the sfz sounds should play through the GarageBand controller, e.g, command-K keyboard.

The plasticmind tutorial points you to the CamelAudio Alchemy plugin, which is no longer available. An alternative plugin is szforzando from Plogue. The Sonatina orchestra sounds in the tutorial are still good.

This certainly helped, but I have to load an individual sound into Plogue to get Garageband to play it. That is, the banks of sounds don't show up anywhere, and aren't accessible from within the Garageband interface.