Make the computer the master, and hook it up like this.
First, find the page in the Triton (probably 'global') and turn 'local on/off' to 'off'. This will break the connection between the Triton keyboard and the sounds inside and is very important, so don't skip this step.
Here's how to connect the midi cables.
Triton midi out to mbox midi in.
Mbox midi out to Triton midi in.
Triton midi thru to MPC midi in number 1.
Go into 'midi/sync' mode on the MPC and set 'sync in' to 1
You will be able to play the MPC both thru the computer and, if the midi track is set up on the proper channel, from the Triton keyboard. The MPC pads are not set up to input to the computer in this configuration, but you can still make beats and sequences on the MPC.