set controller type to "analog" and in the rumble options.. Set them as "directx", big motor to "constant", and small motor to "sine".. This is what I use for my Thrustmaster Rumble Pad and it works great.. MGS still don't rumble though.. Tell me if you get it working for MGS..
When I was using my Wingman Forcefeed back joystick (the same as yours Lugia2002) I had the controller type set to digital/analog in the pad config. If I set it to anything else, the pad wouldn't rumble. Try setting controller type to digital/analog and see if that helps. Also, be sure to check your drivers and DX version, as Bgnnome said. DX 7.0a or 8.0a work best w/that rumble pad.