As Mr June says.
Only there should be no need to pan the mic channel hard left or right. You'll need a lead that goes from two RCA pin jack (your main monitor outs form the Fostex) to a stereo mini jack (microphone input on Mac). Then make sure you check your "Preferences" in Garageband so that it's accepting signals from "Built-in Input" (that's your Mac's microphone input) Which is in preferences under the Audio/MIDI tab, and that the new track you've created to accept the new signal is set to Input Source; Stereo 1/2 (built-in input) and that monitor is switched on.
Then you will get a stereo track with the vocal on it. Of course you can also pan the mic hard left and then set Garageband input signal to "Mono 1 (built-in input) and then pan the track hard left. That way you can actually record two separate signals at the same time by panning one hard left and the other hard right (setting the second GB track to Mono 2 (built-in input). This can be a bit disorientating for monitoring as you record though.
Hope this helps.