It is still using a 2D plane for mouse position so like in the example project you have to imagine the camera/player in a 2d top down sense.
You could use something like:
If mouse is moving* (Need to set up events to detect mouse movement)
if mouse.X < (viewportright-viewportwidth/2) set player angle to self.angle-1
if mouse.X > (viewportright-viewportwidth/2) set player angle to self.angle+1
EDIT: Better to do a distance from centre rather than is moving