Pella is in between Boulder and Longmont, off of N. 75th Street, east of the town of Hygiene. Take I-25 to the Longmont/Lyons exit/CO-66, take CO-66 west probably 6 miles until you get to N. 75th Street South; drive a few miles, through the town of Hygiene, and you'll run right into Pella (on the left side of the road).
OR. . .
Take 75th St. north from Longmont for a few miles, and it'll be on your right. If you've ever been to Lagerman Res, Pella is off of 75th St, just a few miles north from Lagerman.