this link should help you a lot
http://www.do-it-yourself-help.com/wiring_receptacles.html
for a 240 volt outlet you only need the 2 lives, and a ground
you ran a black from each side of the double pole breaker (that makes up the 240 volts) those blacks get connected to the 2 sides of the plug and the bare wire (ground) connects to the ground screw on the plug, the whites are not needed (you ran 2 120 lines instead of 1 240)
use just 1 line, connect the white to the other side of the breaker so you have the black and white wires connected to the double pole breaker (mark the white so you know its live with red or black electric tape) and connect the black and white (now live, not neutral) to the 2 sides of the plug and the ground to the ground screw
ask questions if you dont get it
hope this helped