Found 2 DirectInput devices:

0: 'Keyboard' axes: 0, hats: 0, buttons: 256 (buffered)
   1: 'Mouse' axes: 3, hats: 0, buttons: 8 (buffered)

So let's get this straight:
Left, Right, and Middle clicks work.
Wheel Up and Wheel Down work.
X/Y axis doesn't seem to work yet.

src/arch/InputHandler/InputHandler_DirectInputHelper.h

@@ -23,11 +23,9 @@ typedef struct input_t
 struct DIDevice
 {
 	DIDEVICEINSTANCE JoystickInst;
-	DIDEVICEINSTANCE MouseInst;
 	LPDIRECTINPUTDEVICE2 Device;
 	RString m_sName;
 
-	//enum { KEYBOARD, JOYSTICK } type;
 	enum { KEYBOARD, JOYSTICK, MOUSE } type;
 
 	bool buffered;