If you're working with a servo there should be three wires. The red/black wires are your power for the servo itself. Then there should be a yellow or white wire, which is your signal wire. This recieves pulses from a source and in turn it moves.
To connect the DC motor then, you want to avoid the signal pin, because all it does is recieve the commands. You would connect the motor to the red/black pins. Make sure the DC motor is capable of the power that it gives.