I just recently installed an APEXi S-AFC II. This is a piece of equipment that overrides the ECU to adjust the air-fuel ration at different RPM points. But, i'm unsure about the settings for Pathfinders 3.5 V6 DOHC. ... Does anybody know what settings i can use for maximum HP gain? If not, where i can find this out????

You may have to tune it yourself and figure out where it needs tweaking. Are you sure you're going to get "huge HP gains" out of fuel and timing tuning? I'd dyno it and find out. :) I wager 5-10 HP tops.