Yeah, I don't think a nipple on top is what you're lookin for. If this was where you fill from, your whole pumpkin would be full of fluid. Fill plugs are usually around the halfway point between top and bottom and you fill until fluid is level with bottom of hole.
If it's not on the cover, it probably is on the forward side as rcurrier says and probably does have a square hole for 3/8" socket extension. My Pontiac is set up this way. I had to dig and uncover some dirt to find it.
-David
