My rig: Mac G5 2.66GHz Quad-Core Intel Xeon, OS 10.6.8, Soundtrack Pro 3.0.1
My connection: 1/4″ male to 1/8″ male, between my guitar and the Line-In input on the back of my Mac.
Connection path at the top of Soundtrack Pro:
1. File > New > Audio File (selected Mono Channel at 44KHz).
2. At the far upper right window with tabs “Meters” and “Recording”, I clicked “Recording”. A new window opens, with “Input”…I selected “Built-in Line Input”…your “Monitor” choice is your decision.
Then at the bottom on Soundtrack Pro, click on the red record button and you should be in business.
Caveat: I did not use a USB cable, so that might be your issue.
As a suggested work-around, you could record directly into GarageBand (assuming you have that) and transfer the audio file into Soundtrack Pro for whatever else you planned to do.