Damn kid, making everyone chase their tails to help you on this one
You need to plug your main outs left out into the mixers channel Input left, and the main outs right into your channel input right. Only use the headphone out when you are using a mono input on a mixer etc. so it will sum the left and right channels into one mono channel for you.
I highly recommend doing reading on how mono/stereo input & outputs work. Especially in relation to how it effects volume. I've solved some major issues for folks just by cleaning up their routing or turning a pan knob.
Basically if your using your headphone out (which is a mono summing of left & right) and plug that into one side of a stereo channel. Your gonna have both left and right channels from your MPC only on one side of the mixers left or right. It's easiest to think of stereo audio being dual mono sent to left and right speakers.