It's in the Mendocino National Forest, just north of Clear lake. The riding area is commonly known as Middle Creek; staging at CC Camp. It's got many good trails; I think elevations are around 4,000 feet for most the good stuff, though CC Camp is lower.
Besides being a good enduro and riding area, one reason for a SoCal rider to select this event from the D-36 calender is the date. While fall weather in NoCal is notoriously fickle and early November could be anything from a dustbowl to a snowfest, the odds are good the first rains will have come, yet it won't be bitterly cold. Of course, some years it starts snowing in October, and others it won't rain until December!
BTW, the Cowbell is not at the Cow Mountain BLM area that Holeshot mentioned. D-36 does have a cross country race at Cow Mountain, in the spring.