The Park-n-Ride @ 120th and Wagon Road is the one you want to get to. Go north on I-25, exit onto 120th Ave., and turn left (West). The Park-n-Ride will be on your left (southwest corner of I-25 and 120th Ave.).
For more information, go to http://www.rtd-denver.com/
, click on the "Park-n-Ride" link on the left of your screen, and then click the "Wagon Road" link on the map. That will give you more information, as well as maps of the P-n-R lot.
Hope that helps!