The coolant drain 'tap' is located in the bottom tank of the radiator (normally). Other option is to just remove the bottom hose clamp and slip the radiator hose off and dump the coolant that way. To scour the oil from the cooling system you can try filling it with demineralised water (or rain water depending on how clean it is) and adding a cup of kerosene into the radiator and running it for a while.