At about 1900 Pacific time, a Cessna 172S departed the taxiway after landing and struck a median at Riverside Municipal Airport. The pilot was not injured. The pilot said he landed on runway 27 and was attempting to taxi to the ramp area. He was unfamiliar with the airport and requested progressive taxi instructions. In an effort to comply with what he thought the controller was telling him to do, he departed the taxiway, continued down a grassy area and collided with a median.