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If you have a tank, troop, or jeep of any level next to a transport in the sea, you may "move" that unit onto the transport.  Just click on the unit and use the move command.  You may also use v for a moving shortcut.

You may also build a unit onto the transport if that transport is within a land base's range.

The V move Rick was talking about is hotkeys which you have to turn on in settings in case your not aware you click on the unit and hit the hotkey for it to work