Hi Gene,
There's two options really...
1. Fix a nogging(?) - a bridge between and fixed to, two of the joists (under the floor of upstairs if your projector is downstatirs or in the loft if it's upstairs). The projector then gets screwed into this. It's effectively another joist where there wasn't one before.
2. Use plasterboared wall anchors/toggles. When bought these look like a metal cylinder with a bolt through the middle. The sides of the cylinder are 4 metal strips with a small, perpendicular crease in the middle. The whole assembly is pushed into a hole drilled into the ceiling then the bolt passes through the projector mount into the metal cylinder (now in the ceiling). When you tighten the bolt the top of the cylinder is pulled down and, when tightened fully the metal strips forming the side of the cylinder fold and form a cross (in the ceiling cavity) preventing the bolt from slipping through. Once in - these are NOT coming out
Sorry it's a bit of a long post (and probably very confusing!). Have a look
here to see what I'm talking about. Remember - ceiling plasterboard is thicker than walls so make sure you get the right thickness.
That's all from me now

, good luck,
Matt