Hi,
yeah, now I found this topic which has a clear answer like yours.
The interface was not clear for me, and the game does not have a tutorial
http://www.ghostrecon.net/forums/index.php?showtopic=35200&st=30&p=458067&#entry458067
*****************
Common problem, but it's just an interface issue, not a bug.
When resupplying, click the blank, third tab (to the right of your sidearm) and a list of available extras will open up. The Zeus will be there, if it's available for the mission.
You can also use this third slot for other things, like extra ammo, grenades, etc. Don't forget that you can also equip your team's third slot.
*****************