Upon arrival at his destination airport, the pilot of a PA-30 attempted to extend his landing gear in preparation for landing. He discovered that one of the landing gear system circuit breakers had tripped. He attempted to reset the circuit breaker, but it would not reset. He then attempted to reset it a second time, which was successful, and he observed that the "green" landing gear down light was illuminated. During the landing roll, however, the landing gear collapsed and the airplane was substantially damaged. During the recovery of the airplane, when the master switch was turned on by the pilot, the landing gear dropped into the extended position. During interviews with the pilot, it was discovered that after the circuit breaker was reset, he did not continue with the "landing gear failure and manual gear extension" procedure, which was published in the pilot’s operating handbook for the accident airplane. A postaccident examination by a Federal Aviation Administration inspector revealed that the landing gear would extend and lock in the down position when the procedure was utilized.