On my G5 at least, the usual headphone & microphone combos don't work. I believe you require "line level input". I have no idea what this means, but believe it means a powered microphone?
If the line input is a true line input, and the microphone is unamplified, it could be that the mic signal (which is way weaker than a line level signal) is just not enough to register. You would have to amplify the mic signal somehow.