Is that a NEMA 14-30P? Usually they say somewhere on the plug. Anyway, your neutral (white wire) should be that L-shaped prong. Ground (green wire) should be the round prong directly opposite from that. The 2 flat prongs (black & red wires) are your line circuits L1 and L2.
As JayzAuto suggested, you should verify all this using a meter before making your connections.