There are quite a few ways to access this location but perhaps the easiest (and its approximately a 1 mile walk to and from the reservoir) is off the intersection of Paraiso and Arroyo Vista in Rancho Santa Margarita. The details below explain how to get to the reservoir based on this access point. Alternatively, you may wish to explore the entrance on Google Maps from Olympiad in Mission Viejo at the back end of Beebe Park.
The GPS coordinates of the reservoir on the Arroyo Trabuco Trail are approximately at these GPS coordinates: 33.609333, -117.627315
The trail head and parking at Paraiso and Arroyo Vista intersection are at these coordinates: 33.609333, -117.627315
Simply copy and paste the reservoir or trail head coordinates at Google Maps
I’ve photos below to offer a rough sense of the terrain. Please bear in mind, you may come across Bobcats, Southern Pacific Rattlesnake (both of which I noted on 7-14-14). There’s also a probable stream crossing depending on what time of year you seek out this trail. Photos below were taken mid July 2014. The stream was low so crossing was no issue and was approximately 1-2 inches in height (photos below also)
Bear right at the trail sign, then a quick left and walk down this slope. Head south (left).