There's a couple of things you can check, do you have Derive From Maya in the Frame Buffer for the Mental Ray render globals? Try going down to the animation section of the globals under mentalrayItemsList, uncheck Derive From Maya, then check the Animation box. Now you can set the frames to render from the pulldown and input boxes. See if that doesn't help out
Red bellows of flame have blackened my stones
Convulsing my frame and cracking my bones
Hell's dragons of steel who roar in their chains
Crawl into my caves to suck out my veins.....
-The Mountain P.F.M.