There are several ways to access the cache, but I recommend the trailhead at N 32° 50.430 W 117° 02.277, then just follow your GPS. It's an easy hike a little less than a half mile, and you will be able to check out some Indian grinding holes north of the main trail on the way to the cache.
Since the park has many sensitive habitat areas, be sure to stay on designated trails. As you approach the cache area, there are some blocked-off trails which shouldn't be used. From the main trail, you will be hiking down a small hill on an offshoot trail that is open for use. (Please let me know if you find out otherwise.) In respect for the wildlife and sensitive habitats in the park, the cache is hidden very near the trail and shouldn't require much trampling.
The cache is a medium-sized tupperware with a white lid. Original contents are: $1 bill, Kennedy half dollar, turtle, patriotic button, silly putty, hair clips, dental whitening system, San Diego County map, gold chain, petrified wood, Crocodile Hunter mini light/keychain, and the logbook with pens. Please rehide the cache exactly as you found it or better.
If you're lucky, some bullfrogs will serenade you while you're at the cache (during frog season, that is). And, while you're in the area you can check out the Viewpoint of the Old Mission Dam a little farther west on the main trail: N 32° 50.423 W 117° 02.603.
Just a note: There are signs posting when the park's gates are locked - be careful not to get locked in. Also, be aware of our snake friends, as this is their country.
Have a great time in this beautiful park!