Edirol Fa66 & Garageband Digital Recording Options

[aerog]

How do I get digital sound output from Garageband using the audio interface "Edirol FA66"? I connected my guitar to the interface and the interface is connected to my Mac via Firewire. Mac recognizes the interface, and I can play/record the guitar if I use a real instrument track in Garageband, but I cannot get output at all if I use a software instrument track. I thought that connecting the interface to mac is the same as connecting a MIDI keyboard to Mac(in this case I can use a software instrument track without any problem.)