If it's dry rock you could just bleach/acid wash it (if you are worried about organics or bound phosphates). Leave it in some saltwater with a heater and powerhead for a week or two after treatment and test the water for ammonia/nitrite/nitrate and phosphates just to be secure, then throw it in the tank. The dry rock will become live over time in your tank as the bacteria that comprise your biological filter spread to it.
However, if you are uncomfortable with that you can certainly make it live before you add it to the tank, and then the procedure is just the same as making dry rock live before adding it to a new tank.
The way I go about making dry rock live for a new tank is:
1.) Place the dry rock in a container (the aquarium to be used, a rubbermaid bin, pretty much anything you'd feel comfortable storing saltwater in) along with a couple of pounds of rock already live from an established reef. Make sure the container has flow from powerheads and a heater at the appropriate temperature (I like to cycle at 84 degrees. The increased temperature makes the bacteria multiply more readily).
2.) Dose ammonia up to 5 ppm ammonia using either ammonium chloride or ACE 10% ammonia without surfactants.
3.) wait a few days, then test for ammonia/nitrite/nitrate
4.) repeat the process, re-dosing ammonia every time your level drops to 1 ppm until your rock can process 5 ppm of ammonia to nitrate in 24 hours, leaving ammonia and nitrite at 0.
Congratulations, your dry rock is now live with a healthy and robust biological filter. Feel free to add it to your tank!
(procedure adapted from Moe, Martin. Marine Aquarium Handbook: Beginner to Breeder. Neptune City: T.F.H. Publications, Inc., 2009.)
