Yeah, MPC has Line and Mic input, but it doesn't have Hi-Z, which is why you can't hear anything. Best to hook it up to a preamp between the guitar and MPC.
And if you want to play over backing tracks strictly on the MPC, then just go into Record Mode and hit Play. You'll hear your guitar and a sequence will play in the background. Unfortunately for that method, you can't change sequences on the fly, so you'll have to stop everything, get out of Record Mode and set the next sequence. If you don't mind then go for it, I do recommend a mini mixer to do the job, so many options out there and for low as $20 bucks.
P.S. In Record Mode you can record your guitar performance and it'll be a lone seperate sample without the backing tracks.