If you have an external mic plugged in and your computer can see it, then it should show up in your microphone list. In the case of my setup here, I have an external USB microphone called Doubler USB, and I select that, which makes it so that only that one will be used with TalkTastic while it is available.
Tips:
If the external microphone that you had chosen is no longer available (because it was unplugged), it defaults back to "Same as System" and chooses whatever the system chooses.