Nothing. You don't need it. Wire your blues together to L1, white to L2, and don't worry about the color extender. You don't need it. If you were doing RGB, or wanting to throw a 3rd color in the mix, you'd need it. B,B,W. Just wire both blues to L1, and white to L2. Simple.
The color/power extender is mainly used as a way to get another mosfetted board to drive a 3rd color. There was only enough room on the NBIV for 2. Adding a 3rd would have made the board bigger. So they added pad 3 to power the exender if you needed it. On your current LED, its not needed.
Tom