That's brilliant, except now you have to figure out the code. That shouldn't be too bad if you know how to program this controller. But it you don't, best to just buy the kit, which should give you access to the firmware. If you don't have a serial port, there are a myriad of USB to serial convertors that you can use in conjunction with terminal programs. Alternatively, you can use products like those from Wiznet to control your relays directly using USB. Your time is better spent learning to use these products rather than trying to come up with your own USB interfaces.