Hey, I have a issue. I have Canon 5D Mark II, and my microphone's working is very unpredictable. Sometimes it says "microphone unconnected" when microphone is inside the plug. And if it is detected, it records only right channel. The microphone has TRS-cable, so I'm helpless with it.
Have you tried the microphone in something else to make certain it's working properly, it will probably work with a laptop.
I know there have been problems with the 5D mk2 mic input where a connector inside the camera has come loose. I remember reading about this in a thread on the Dpreview forum some time ago.