Is one of the new versions, it says you can use multiple STBs. Which I guess means you can use MC to watch stuff through an STB, like you can with the XBox One.
Take something like Sky+ HD in the UK. How would it work for On Demand stuff? Series Links, etc. etc.? Does it literally just pass the signal, which would require an HDMI input on the PC?
As Alex said, you need a capture device that will take the signal from your Sky+ HD box. If your STB has HDMI output only, you will need a capture device that has an HDMI input. Otherwise, you can use Composite, or SVideo etc.
You will need an MCE remote control with IR blaster that will send channel-changing commands to your STB.
MC will run the capture device for showing or recording programs. It will also send IR signal to STB to change channels.
MC does not directly deal with On Demand. In other words, MC can only let you tune to the On Demand channel, but you will need to use the remote control of the STB to make your On Demand choices.