After obtaining your visitor tag, proceed through the gate, stay on the same road, until you get to the Pendleton and 41st Division intersection (should be the 3rd light after entering the gate - on the right is the Post Exchange).
Turn left at the intersection, and as you proceed, Pendleton becomes Jackson.
Proceed on Jackson to the Medical Mall entrance to Madigan, turning left into the Medical Mall parking lot.
Enter the Medical Mall. Dermatology Clinic is just behind the escalators.