Now that you've wired everything up and added your board to WipperSnapper, it's time to set the sensor itself up.
Call it whatever you want. I chose to call it door, but anything else should work as well. Set Reed Switch Pin to D27, make sure that Specify Pin Pull Direction? is selected, and set it to Pull Up. Also make sure that Return Interval is set to On Change.
Now, as you open and close the door, you should see the grey box in the sensor box go from "Open" to "Closed."