I'm sorry, but I'm stumped. If a microphone was plugged into the microphone port I don't know why it's not showing up in Audacity. (Is it plugged into the microphone port, or it a USB device?)
You probably have already gone through this, but the bottom of this page covers "Selecting Recording Input" --
http://web.stanford.edu/dept/lc/ats/...structions.pdf
And, someone in the Audacity forum may be able to help --
http://forum.audacityteam.org/