(responded already to your email, just posting here in case it would help anyone else)
You can rebind controls in the "Options" -> "Rebind Controls" menu.
The game should recognize that you want to use the mouse if you simply click anywhere in the window. I tend to right-click because a left click will fire your current weapon.
If you have a gamepad plugged in that you don't want to use, or some kind of USB device which could be construed as a gamepad, I suggest you either unplug it or make sure it's in some kind of state where it's not going to provide any input. If any input occurs, the game assumes you want to use the gamepad.