As he was preparing to land, the contract pilot, who was positioning the airplane at the destination airport for minor maintenance, inadvertently set one number incorrectly when he initially set in the common traffic advisory frequency. After transmitting his position/intent four times, and hearing no other aircraft on the radio (including one that he saw on the runway), he realized that his frequency was not set correctly. He therefore set in the correct frequency. About the time he was resetting the frequency, he lowered the flaps to full, but inadvertently forgot to lower the landing gear. As he continued on around the pattern, he became "preoccupied" with the aircraft that was on the runway, and did not notice that the gear was not extended. He subsequently landed with the gear retracted, resulting in substantial damage to the belly stringers and bulkheads. The pilot reported that there were no anomalies associated with any of the airplane's systems.