The entire head (and therefore the lens stage) can be tilted forward and back by rotating the axles for the guide rollers. The holes in the ends of the axles are drilled off center and there is a hole in each axle perpendicular to the main axis that you can put a small hex key or screwdriver in to rotate it. Be sure to loosen the screws a bit first. The lens stage can be angled left and right by loosening screws at the bottom of the focusing mechanism and sliding it left of right (what it looks like depends on your model). You already know how to adjust the negative stage.
Search around on the units and I think you will find the adjustments quite easily.