On the FMX, it's on the back driver's side of the main housing.
It looks like a medium sized gold mushroom with a vacuum hose going to it from the intake manifold. If you pull the hose off the modulator, inside the stem of the modulator, there will be a small allen screw. Turn yours counterclockwise about 1 to 1 1/2 turns. Reconnect the vacuum hose and go for a drive. It should affect your shift points, not a great deal, but perhaps enough. Continue to adjust as nessesary.
Same size, shape, adjustment, vacuum hose, etc., but on the passenger's side of the transmission.
Always check the fluid levels when troubleshooting any kind of hydromechanical unit---power steering, transmission, rear differentials, etc.
It sounds obvious and stupid, but almost 50% of the times it turns out to be something simple.
you can manually keep it in gears 1 and 2 as long as you want
but watch out, i blew out my 2nd gear clutch on my c4
if your on the highway, throw it all the way down, and the tranny will shift into second
if you have it in low from a start, it wont shift out until you do it
but what i did, was id shift into second, then put the shifter back in low and it would stay in second gear as long as i wanted
this will really help your acceleration, by making the transmission kick down