I don't use GMax, but the concept is probably the same. You need to make a helipad and place it at the same co-ords as your building with the same heading and at the same altitude as the height of your building. If you want the building's roof texture to show, then don't assign a texture to your helipad. This will give you an invisable landing surface. If you want the whole roof to be landable, make the helipad the same dimensions as the roof.
This is hard to do with default buildings, but if you designed the building, you'll know what the exact dimensions and co-ords are.

::Helipad is a marked/unmarked place to land/takeoff helicopters, a roof of a building, forest clearing, anything can be a helipad.

I Dont know scenery design well in Gmax, but a helipad can be easily made with VOD.

???...and I think EOD can as well  ??? Just not sure.

Just a suggestion: As when designing aircraft in Gmax, we assign various parts with appropriate names;
i.e prop, prop_blur, wheel, elevator_r, etc...and landing lights the same way by naming a polygon to be, and let MakeMDL do the rest, maybe a helipad can be done the same way!? Just dont know exactly how. But I suspect that the MakeMDL module does that.

All the best!
Anish

VOD is a difficult program for beginers, i suggest making your helipads with EOD http://www.echos.ch/eod/index.html Macros of anykind are quite easy with it.