Um, well have you looked around the app?
Basically, you create a context for each situation, so one named "internal display" and one "external display". Then you enable "Monitor" under "Evidence sources" so MarcoPolo knows what to look for.
Then go to "Rules". Now create a rule for every situation like this:
Click the plus, "Add Monitor Rule", select the the internal montiro from the list (it has to be connected at the moment, though), then select the matching context and set the confidence to 100%.
Repeat for the external monitor.
Under actions, click the plus, "Add Open Action" and then choose the System Preferences app. Choose one of the two contexts and "on arrival". Repeat for the other context. This should be enough.