The pressure switch will not have a receptacle on the outside of the spa pak.
I am not intimately familiar with your product, but I would bet you have J&J plugs for you pump(s), ozonator, light (s), blower etc. You will also notice that the heater is not "plugged-in" the same way.
I would bet your pressure switch connects directly onto your lower board rather than plug-in.
If you are lucky, there will be a schematic on the back of the door to the spa pak. This may indicate where the pressure switch attaches to the board. If not, call a the manufacturer or a local technician. You need this switch plugged in and functional.