there are two methods
(note - this is covered in the pfc faq)
method #1
the powerfc has the 5 pin boost kit plug
you need to work out which wires are for the solenoid and which are for the map sensor
i dont know which ones are which, i only know the pinouts for the 3 pin kit
the only thing i can think of to help here if you do have the 5 pin kit is buy the 5 pin kit harness
its only $30 from nengun.com and it will save you guessing wires and risking blowing up shit (or the ecu)
method #2
the powerfc has the 3 pin boost kit plug
this is easy - the 3 pin socket is the map sensor.
the solneoid will get its power from the standard solenoid (usually listed as wastegate solenoid or valve) or the cars wiring loom.
this ONLY applies if the car is fitted with a standard solenoid to run a hi/low boost style setup from the factory.
the GTST skyline and GTR do, they both have a solenoid to give you dual stage boost. if you car does not have dual stage boost, then your PFC will be the 5 pin kit
with the map sensor on your WRX if you do MONITOR, 1 CHANNEl, BOOST, does it come up? or is it something else?
can you take a photo of ETC, SENSOR SW CHECK