Mx vs atv alive says activate second controller
I guess it's as easy as pressing A or Start.
But if the controller really isn't connected anyways, you have to:
1: Turn on controller, and hold the sync button ( just to the right of the Left Bumper ).
2: Press the Sync button on the Xbox 360
> Slim version: Sync button is placed just beside the usbports on the front of the xbox.
> Original version: The sync button is placed just beside Memory Slot B ( looks pretty much as the one on the xbox controller.
3: The controller should connect and sync automaticly to your Xbox 360.