MPCs are samplers. MPDs are controllers. If you have an MPC, you can sample from almost any audio source. Just plug the lines into the MPC and record. However, you can't record samples using an MPD because it is a controller, not a sampler.
Yes, you can use the MPC to sample from an audio track on itunes. You can use your computer to playback the audio file and then use the MPC to record the samples you want. Or you can use an audio editing software to "chop" the audio file into samples and load them into the MPC. The first way is probably the easiest since you will be recording the sample chops directly into the MPC.
I have the Akai MPD32. I don't have the MPC. I use the MPD32 to trigger samples that I have already loaded into the devices in Propellerhead Reason.
Let me know if that helps.
Grace and Peace!