http://usfirst.org/uploadedFiles/3-F...s-Rev-0-5a.pdf
Per chapter 3.5.3, there are 10k pull-ups to 5V on all of the sidecar's GPIO. You should be able to simply attach the pressure switch to the "signal" and "-" terminals for GPIOx (the outer two terminals in each GPIO's 3-pin set, I don't remember which one is intended to be used for pressure control).
Russ