To make a chopper land and wait for units to get in: Give the infantry a get in waypoint. Give the chopper a load waypoint. Synchronise these two waypoints.
To get the chopper to land where you want it to while this happens could be the subject of several PhD theses. The first step is to use an 'H' object. If that would look odd use an 'invisible H' object. Think of these objects as an invitation to the chopper to land there. If you place this inviatation on flat ground, it might well do that, or it might find a more desirable place to land some distance away, if it is placed on sloping ground then it depends on the slope as to whether the chopper will ever accept the invitation. To make it land on the H, what I have done in the past is to give the chopper a series of waypoints, all limited speed, all close together, the first has choppername flyinHeight 50, the next has choppername flyinHeight 45, then next choppername flyinHeight 40... until choppername flyinHeight 5 then the next is the Load waypoint. At a height of 5 the chopper is then unlikely to be able to find a place to land that it prefers to the H object you have placed on the map. Probably.
You need to experiment with this - sometimes a lot.