As I mentioned before any sensor would work. Here I just show board layout because some people (especially those who came from .NET world like me) have troubles with wiring. PIR sensor has three pins: one for voltage supply, one for actual detection and ground.
Connect voltage supply to 5V pin on Netduino Plus board, signal pin could be connected to any digital pin (I prefer D0), ground PIR sensor lead connect to (surprise, surprise!) ground pin on the board. For true electrical engineers I also show layout schematic below.