geoff12345: ok, I am puzzling my way through this, thanks for all the comments.
When the yellow bar under the supply unit designator reduces to black, does that mean the supply unit is out of supply? (Which would explain why it won't re-supply my reconnaisance units :)) In this scenario, I only have two trucks. It sounds like that is not a lot of supply.
How is a supply depot, or FOB I think the manual calls it, marked on the map?
An FOB is a big structure full of crates, containers and barrels. You won't see them for a while though, the first campaign is actually a tutorial for the, well, real game. It will kick your ass though, this is first-day-at-law-school levels of tough. The proper campaigns are even harder.
And yes, once the yellow bar is out, the truck has no more supplies. You can still call in new ones, but you don't really need it in the first mission.
The first 2-3 missions are not going to give you full access to the game, they slowly introduce new gameplay mechanics throughout. It isn't a good tutorial by any stretch of the imagination.