You're going to have a few problems with that mouselook code:
1.) The mousemove commands have no spaces and no parameters -
mousemovex(),
mousemovey(),
mousemovez()
2.) Z-rotating the camera will basically cause it to turn sideways - just x and y rotation should be enough.
3.) The mousemove commands return the distance in pixels the mouse has moved on the screen, as you probably know. However, this also means that if you just rotate the camera directly with those values, a little move is going to take it a long way. Try dividing them by 15 or so.
4.)
Rotate Camera does not rotate a camera more along each axis - it sets the angle of the camera at each axis. Therefore, for this to work as you want, you need to add each value to the old camera angle. It's the same with
Rotate Object and
Rotate Sprite.
See if this code works for you:
rotate camera mousemovex()/15 + camera angle x(),mousemovey()/15 + camera angle y(),0
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