I am struggling with this one too Jeph. I think the folks who were in charge of this area of GB 10 were sleeping or maybe doing something else. The hillarious thing is that the help file tells us how to use this feature...
1. Double-click the track’s header, or select the track and choose Track > Show Track Info.
Well, as much as one tries to figure this out there is no Show Trek Info submenu on the Track menu and double clicking the track's header lets you edit the name of the track. Nice. This bit of help from the lovely helpful seems to be relevant for GB 6. Not GB 10. It is kind of a give away in that this page of the help file has a 2012 copyright on it. Sigh. I guess no one can be perfect. But this is a bit annoying.
This is super simple to do in GB 6. There is an Edit are for each instrument and you can do anything you want there. GB 10 seems to have removed that option. Quite annoying.
If your soundfonts come as Audio Unit plug-ins you can add them from the Smart Controls:
but make sure the audio units are compatible with GarageBand 10.0: GarageBand for Mac (v10.0): Unexpected behavior with Audio Units ...
Open the GarageBand Help and go to the section "Use Audio Units plug-ins"
You can add Audio Units plug-ins to a track (including the master track) in the Smart Controls pane.
By default, the Audio Units area is not visible in the Smart Control inspector. When you open a song that uses Audio Units, the Audio Units area is available in the Smart Controls pane, if Audio Units plug-ins are installed on your computer. You can change the Use Audio Units preference in the Audio/MIDI pane of GarageBand preferences. For more information, see Audio/MIDI preferences.
Add an Audio Units plug-in
Click an empty part of the Audio Units area, choose Audio Units, then choose a plug-in from the submenu.
The plug-in window opens, showing the plug-in’s controls. Some plug-ins include a Details triangle, which reveals additional controls.
If the track includes built-in effects, an alert appears, asking if you want to keep the existing effects, or use only the added Audio Units plug-ins. If you bypass the built-in effects, the Smart Controls for the bypassed plug-ins are disabled. Removing all Audio Units plug-ins from the track re-enables the built-in effects and their Smart Controls.
What the help not does say, is, that you have to enlarge the visible area of the Smart Controls to show the hidden audio units. Drag the dividing line between the smart controls and the track area upward, until you see the audio units appear.
Happy New Year 2014!