You can apply all kind of transformation options for the loaded videos once you get them to your timeline.
The procedure is as follows: add the video from the Clip Library then drag it on the timeline. Activate Visual Properties and locate the Z axis (as shown in the image). That's the degree angle that your video will be rotated inside the canvas. If the canvas is too bit, you can increase it using the button on the upper-left area of the Camtasia application. Input the angle in the Z field and the video will rotate. The aspect ration button enlarges or reduces the size where the video fits. Make it a little bigger to be able to rotate the video while keeping all the elements visible.