I have used several external interfaces with Camtasia for Mac.
They work fine, and so should yours -- except for recording STEREO audio!
Somehow, Techsmith can't figure out how to record stereo with Camtasia on a Mac, even though with OSX Core Audio, it ought to be a non-issue.
Oddly, I have seen Techsmith Jing on Mac capture stereo audio, as well as other screen video capture tools.
I'm not sure if it is just because Techsmith does not care, or if they have not hired someone that can make it work, but it has been an ongoing problem for several years -- since the original Camtasia:Mac was released.