Leaks where it sets on roof how does it bolt to

hiltonjug is correct with his answer but if the weather stripping just wont seal or is bad the cheapest fix is to install some 3/8" shims about 2" wide under the roof and on top of the vent frame to raise the unit and stop the water from pooling under the unit. The drain hole on the pan is derectly behind the seal and if there is a depression from the ac it will collect around it and eventually drip inside the ceiling assembly.

There is a bolt on each corner that tighten from the inside. You will see them when you remove the inside cover. There should be foam weather stripping around the opening that the unit sits on and then it's tightened up.