Ok, put on your safety glasses. Make sure your gun is unloaded. Check and make sure your gun is unloaded again!
Make sure the hammer is NOT cocked. With a 1/8" punch, gently tap the pin out from the hollow (dished) side of the pin. Once the pin is out, you should be able to slide the mainspring housing out of the frame. If it is tight coming out, cock the hammer. The hammer strut will push down on the mainspring, allowing the housing to slide free.