When you turn on Camtasia you should see the option to "Record the Screen". Select this and a small window should appear. Look for the box labeled "Recorded Inputs". There's a webcam and microphone setting. Make sure under the microphone it says "Audio On" with a green check. Now click the down arrow to the right of the microphone, and select the microphone you want to use. If none of the names are specific enough to tell which is which you might just have to test each one.
If none of these mics work, you might have to activate it in windows. First go to the control panel.
-Windows 7, click start then click control panel -Windows 8 or 10, search control panel from start menu
Go to Hardware and Sound > Manage Audio Devices > Recording
Find your microphone, right click and set as default. You can find your microphone by talking into it, and looking for the one that lights up green.