Stereo Input with Sony Microphone

I am using garageband to record live guitar/vocals with an external microphone - Sony ECM-MS907. I plug in the mic, change the audio input under system preferences to 'line in-audio line in port,' and open garageband, but then the only audio options are a) mono 1 - built in microphone, b) mono-2 - built in microphone, and c) stereo - built in microphone. There is no option for an external mic. The mic I'm using records, but not in stereo. What am I doing wrong? How do I make my recordings in stereo? Thanks!