From Moab, take Hwy 191 north to state road 313 west for approximately 22 miles to the entrance of Canyonlands National Park Island in the Sky District. Travel 6 miles past the visitors center to the trailhead. It will be on your left. The hike from the parking lot to the arch is a mere 500 yards.
Mesa Arch, located in the Canyonlands Island in the Sky District, is a "must see" for park visitors. I've been told it's a nice visit at sunset, but my preference is sunrise. For a few brief minutes the sun illuminates the interior of the arch and sets it aflame in hues of red and gold. I recommend coming early as this tends to be a popular spot amongst photographers and real estate at the arch is limited. If you're staying in Moab, MAKE A HOTEL RESERVATION or you may find yourself sleeping in the backseat of your car! It's a busy little town and rooms fill up fast. Give yourself about an hour to reach the arch from Moab.