I have a 1992 nissan d21 pick-up 4WD, 6 CYCLINDER and am trying to add gear lubricant so it will stop slipping how do i do that

I am not sure if you mean transmission fluid, or gear oil. I will assume that you mean transmission fluid, since the level of gear oil in a manual would not affect the transmissions pulling ability. You will want to make sure that when you put the fluid/oil in the transmission that it will not cause it to be over full, so if needed you will have to drain out some fluid/oil before you add the new fluid/oil. In the automatic version there is a dipstick on the passenger side and you can fill the transmission there, and if for some reason you are referring to the manual transmission, there is a bolt that unscrews out of the side of the manual transmission, and that is where you fill it.