Apple's free music creation software lets you host VIRTUAL GUITARIST as an AU plugin, this is a bit tricky because you have to go some extra steps.
1. Choose GarageBand > Preferences…, then Audio/MIDI > Effects and ensure Audio units is checked.
2. Select a Software Instrument track, or create a new one. (Track > New Track… > Software Instrument and click Create.)
3. Select the track, and click the Smart Controls button on the toolbar (the one that looks like a knob).
4. Click the ‘i’ icon in the Smart Controls pane. (The “Inspector”)
5. Look for the Audio Units section – you may need to scroll or resize if it isn’t visible.
6. On the right, you’ll see areas labeled AU Instrument. Choose UJAM and in that menu pick the Virtual Guitarist you want to use.