Okay, I've lived just outside of Grass Valley for 15 years now, and delivered furniture for a store in GV (that's a great way to learn a town), so here's some directions:
take Hwy 49 north from Auburn.
when you're coming into GV take the second offramp, it will be Hwy. 20 to Penn Valley.
at the end of the offramp is a light, take a left, you are now at the beggining of Hwy 20
take the first offramp, it's about 1/4 mile
the offramp will loop 180 degrees to a stop sign, take a right, it will take you under Hwy 20
50 yards from there you will come to a stop sign with a yeild sign on the right turn, turn right
now you are on the right road (McCourtney), go about 1/2 mile and the fairgrounds are on your right, can't miss them, just be sure to know the gate you're looking for.
ACTUALLY... I just had another idea to make things simpler, and maybe faster, maybe slower, but not either by more that 5 minutes.
Take Hwy 49 north from Auburn.
About 1 mile before the Alta Sierra stoplight, or one mile past the Lime Kiln stoplight, take a left on to Old Auburn Rd (might just say Auburn Rd.)
Stay on it for, oh, maybe 7 or 8 miles, This will dead end on McCourtney right at the fairgrounds.