It sounds like you are trying to "live sample" an audio source or track with the MPC plugin. To do so you would need to be able to "monitor" the audio signal at the plugin in order to be able to record it.
I don't have MPC, but I tried to setup the TX16Wx plugin sampler in Sonar/CbB to sample an audio source, and could never get that to work. The sampler module in TX16Wx has input meters to listen to the audio input signal with, but they show nothing when used in this DAW. You can drag and drop samples into the sampler from another track just fine, but cannot record them directly. I tried patch points and aux tracks, without any luck.
I think the problem is that if a plugin is programmed as an instrument, Cakewalk cannot route audio input to an instrument plugin. There may be cases that might work if the plugin can also be recognized as an FX plugin when used in an audio track FX bin. FX allows for audio input. But I have had no such luck with my sampler plugin in Sonar/CbB.
I was able to use another DAW (Reaper) to make my sampling setup work, where tracks are not defined explicitly as audio or instrument, and the routing is flexible (sends and receives) so that you can route just about anything anywhere.