1. In GarageBand, choose GarageBand Preferences.
2. In the Preferences pane, click Audio/MIDI.
3. Select Symphony64 or Symphony I/O: USB Audio (depending on which mode you're in) in the Audio Output and Audio Input drop down menus.
4. Show the Track Info pane by double-clicking the track icon, then click Real Instrument.
5. At the bottom of the pane, set the Input Source to the desired Symphony I/O input (or input pair).
6. Set Monitor to On.