[Imp]

Unsure what you are saying
In the deployment screen you can use the load all function though you are generally better off loading units individually if you want to be sure which vehicle has which passenger.

In battle use the L key to load/unload (about 4 buttons from top I think if you want to use the interface)
Except in rare cases like landing craft the unit must be in the same hex as the unit it is loading onto.
If the vehicle already has a passenger make sure you have the unit you want to load selected when you press L otherwise you will unload the current passenger.
If it wont load its because it wont fit, see the game guide for more info on that.