Are you sure you got the right part? As I recall, there are two valves that differ only by thread type. The one for the engine block has straight threads (might be BSP, don't recall) and takes a fiber washer for sealing. But the one for the radiator has tapered NPT threads and the taper provides the seal.
From your photo, though, it looks like the plug part of the valve is what is leaking. Should be easy to fix, just disassemble and remove any burrs from manufacturing, then lap the plug into the body a bit with a light abrasive. Rubbing compound (for paint) will do if you have it, or even white toothpaste.