ive never done it before either, but if you hang em from the ceiling id say get hooks to screw into your ceiling and hanging chains.... you can get the stuff at home depot..... if the pendant doesnt already have hooks or loops on it for hanging youll need to get them too..
I hung my pendants from the ceiling. Just buy a couple of anchors (I like the self tapping kind) and hooks that will fit into those anchors. Then buy yourself enough chain to hang the lights the correct height. I did it all really quickly and it worked well. You may have a hard time connecting the lights to the chains, I used wire and ran it through some fittings on the lights, but this is something you'll just have to figure out on your own as it's hard to explain.
Also, be sure to cover the tank when you're installing the lights, the self tapping anchorse will cause dust to fall.
Unless you have a high ceiling like mine... in which case you're going to need some kind of support rig... personally.. I say build your own. If you build it yourself, you can build it as tall as you need to. And don't custom order it from some guy who says he knows all about how metal halides need to be mounted. Those guys sometimes lie.
Um... wow... where did that come from? LOL!
Seriously though, hanging them from the ceiling is a great idea, especially if your pendant is of a large design and you have easy access to mount it from the ceiling. Make sure those hooks you use on the ceiling are mounted in studs/structural supports of some kind, so the load is easily taken and you don't have the pendant crashing into your tank.