The pilot receiving instruction and the certified flight instructor (CFI) reported that while attempting a simulated engine-out approach, the pilot failed to lower the airplane’s landing gear. During the landing flare the stall warning horn activated followed by the activation of the gear warning horn. The gear warning horn had not activated during the power-off approach. The CFI immediately took the controls and attempted to add full power, but the pilot's hand on the controls prevented him from achieving full throttle application. The CFI was only able to add approximately half power. The airplane subsequently landed in a gear-up configuration. The operator of the airplane indicated that there were no known mechanical malfunctions or failures with the airplane. The pilot stated that he had not specifically checked the gear warning horn on that flight, but it had been operating properly earlier that day.