Not the very best 'where can I find it' photo but:
This is located at the front of the tranny, just behind the Y pipe (yes, it's on a N/A Impreza RS
). Start at the tranny mount and go forward on the lowermost part of your tranny until you find the big nut- it's the only one there. I believe it is in a different location on the Auto, as there are seperate reservoirs for ATF and front differential fluid on the auto tranny whereas they are combined on the manual.