This isn't a trail so much as a series of washes and rock slabs that makes for some interesting biking. While there are a number of prescribed routes, this place is really a free for all. Not all the designate trails are marked so it's best to go with a local or plan for a long day. This is very dry country, so bring more water than you think you'll need. It's easy to get kind of lost but impossible to get completely lost as you can always see White Mesa to the south and your car is between you and White Mesa.
From I-25 North of Albuquerque, take the exit for 550 towards San Ysidro. About 24 miles from I-25, 550 curves West around the south of San Ysidro. Continue west North into San Ysidro another mile and a half and look for a pullout on the right. If you see a gate and signs announcing the Motorcycle Trials Riding area, you're in the right place. Hop on the bikes and follow the sandy road north to another parking area (for members of the Trials Riding Club only) and start exploring.
http://www.singletracks.com/item.php?c=1&i=4502