From I-80 take the Napa Sonoma exit that goes on to CA-12. That'll take you through wine country where eventually you'll hit Napa Valley Hwy. Take a left and continue as the road turns into Broadway Street. Then take a right on to Rio Del Mar, and then immediately take another right on to Eucalyptis (it's nearly a U-turn it's so close), and then follow it through a few stop signs until you see signs for Paintball Jungle on the right.
Upon reading good reviews for the course I decided to try it out for myself and found it to exceed expectations. My experience was very enjoyable and I'd recommend the course for anyone.
The staff was helpful and polite, the fields were well setup and well taken care of, and the Beeswax game was a ton of fun.