You didn't say what version of Windows you are running. If it's Windows 10. What I can tell you is Camtasia 7 won't function properly on it. There are several issues with the program, that when run in Compatibility Mode. Cannot be corrected. From my perspective, 7 has to many issues to be run on Windows 10.
So, being that Camtasia 6 is an even older version. Designed for Windows 7 or older versions. I'll let you draw you're own conclusions as to it's compatibility.
You could try changing the hot key for the recorder.See if that helps.
You may be able to change a configuration in Windows or the recording toolbar to get the recorder to recognize the mic?
Aside from that, upgrading to a newer version of Camtasia may be your only option.If your computer is compatible with a 64bit program.