Here are the directionis from their site:

Directions: Cedar Hill is located 10 miles southwest of Dallas, 4 miles southeast of Grand Prairie, and 3 miles west of Cedar Hill and is accessible via FM 1382. From US Highway 67 exit FM 1382, 2 1/2 miles north on the left. From Interstate 20 exit FM 1382, 4 miles south on the right (on the Joe Pool Reservoir). The park is skirted by FM 1382 and Mansfield Road.