First of all, don't freak out...perhaps you just have a big air pocket in the system that needs to be burped. Here are instructions on how to bleed the system:
http://pedrosgarage.com/Site_3/Install_3rd_Radiator_%26_Coolant_System_Bleeding_I nstructions.html
Check for leaks at the front of the car where the radiator was replaced and along the center section of the car (where the water pump is), as well as by the pass side rear wheel well (to see if the leak is near the coolant tank). It's most likely one of these locations where you are losing your coolant.