External mic not working.

Assuming you have connected the mic to the correct port on the computer (one is for headphones and one is for audio in), you should be able to use System Preferences > Sound to select "Line In" as the input source and then see the input signal register in the meter when you speak into the mic. Are you getting any signal?

Oh yeah. MacBook. My bad. In the Sound Settings window, is there a drop down menu "Use audio port for"?

This is no longer possible on newer Macs, if they have a headphone port, but not a dual input/output port.

If you just want to try out an external microphone you could use an input splitter and connect it to the headphone port, (for example this 2x 3-Pin 3.5mm Headset Splitter Adapter - M/F

http://www.amazon.com/dp/B004SP0WAQ).

Or use the iMic adapter (Recording with iMic in GarageBand). But both will not make you happy for a long time. The audio quality is not that good.

For better results connect an audio interface like the Apogee Jam to a USB port.

Check your microphone input port by going to the -menu, then "About this Mac".

In the "About Panel" click the "System report" button.

Then click Hardware > Audio.

Is your Microphone port listed like this?

Then you need an interface or input splitter as described above.

Or try to return your microphone and exchange it for a USB microphone.