You click on a soldier then hold down the x key, you should see a eye symbol appear while holding down the x key and placing your cursor over either a storage pile, vehicle,gun or other soldier etc. You then simply click whatever object while it gives you the eye symbol. Your infantry man will walk over to the object, then once there, a inventory screen will appear (one thing I don't like is the game doesn't autopause while in the inventory screens so you have to be fast while under attack as the mission hardly gives you any time to supply let alone even position your peeps). You can trade items, BUT the tanks and antitank guns etc all have differrent calibres of rounds. I believe the sherman and matilda is 76mm while the antitank gun is 35mm ? lol, anyway you get the point. Click on the rounds you wish to take until your inventory is full and don't forget to get machine gun rounds for your sherman at the start

. Once you have whatever inventory you want (rounds etc) then again hold down your x key and click on the eye symbol that appears over your antitank gun etc. He will walk over and again a inventory screen will appear then you just offload your rounds into the antitank guns inventory. YOu can also check the inventory of destroyed tanks,cars,jeeps etc for useful items especially repeair kits to fix that tiger once you destroyed it. One note with the game is its easier to kill the tiger with 1 hand held infantry anti tank gun (with the tungsten steeel rounds) than with 2 tanks and a artillery anti tank gun all firing armor piercing rounds (game glitch ?-needs to be fixed bad)I also did not notice any differrent effect by shooting the tiger in the butt with my matilda armor piercing rounds so there is no armor thickness calculation variations that I could see which should be fixed. You can however, shoot the tank in the tracks and make it immobile

To use a gas can or repair box , you move it from the inventory to the soldiers hands then you will see 2 gears as a symbol on items you can repair. For the gas can you will see a cup with a water drop symbol to take gas (it holds 20 units of gas max) and a funnel symbol to fill a object with gas.Note that since any resupplying is slow use 3 infantry at a time to speed it up.Hope this helps.
P.S. For more info The included "readme" that came with the demo tells you what keys to use to do things.
enjoy the carnage !