I don't have my MDX (yet), but i'm assuming it's the same mechanism as on my Integra (and on all Hondas/Acuras). As far as I know, you can't turn off the light yourself. It's programmed to turn on when it's time for a scheduled service. (15K, 30K, etc. or whatever it is for the MDX). Unless there's some trick I don't know of, only the service mechanic can hit the switch to turn that light off when the service is done.
But if anyone knows how to circumvent turning it off w/o taking it in for service, PLEASE DO TELL! I'm due for a 90K service on my Integra, but since I'm about to trade it in anyday now when the X comes in, I'm not about to pay the $550 to have it done
Take another look at the owner's manual. I don't have one handy, but if it works like the TL, it comes on every 6K miles unless it's reset when you have service. As I recall, you have to press the two buttons before you start the car, and keep them pressed while you start it and hold them until it goes out - 10 seconds maybe. Like I said I don't have a manual in front of me so I might be a little off here, but I've reset our TL every time it comes on. I can't imagine that the TL could be done by the owner, but the X couldn't.