Kevo
Chieftain
First of all, you need 3 things: an army with at least one unit loaded, a transport ship with available space, and a coastal city.
Place the army and the ship in the city.
Select the army, and then click on the unit to unload (in the army window).
Press the load button (or L key).
There should appear a window for choosing what to load the unit into, and it should contain at least the army and the ship.
Select the ship and press OK.
Now the unit is loaded into the ship, and it will behave per normal. Unload the unit from the ship and use it as you will.
If there are more units in the army, repeat the above procedure for each unit you wish to unload.
By the way, this works in PTW v1.14f.
Enjoy,
Kevo
Place the army and the ship in the city.
Select the army, and then click on the unit to unload (in the army window).
Press the load button (or L key).
There should appear a window for choosing what to load the unit into, and it should contain at least the army and the ship.
Select the ship and press OK.
Now the unit is loaded into the ship, and it will behave per normal. Unload the unit from the ship and use it as you will.
If there are more units in the army, repeat the above procedure for each unit you wish to unload.
By the way, this works in PTW v1.14f.
Enjoy,
Kevo