I would use a stud finder to locate the register, a friend of mine installed some hardwood flooring and did the same, he covered up a vent, and I managed to find it no problem with my stud finder. The one i have i got from Home Depot, it can detect different density and finds the edges of studs etc. once you have a rough idea of where the vent is, just drill a hole and follow the contour of the vent with a jig saw. I hope this information shall be of benefit to you, and good luck.
Paul.