If you just installed the Weapon for FSX, then added it to the aircraft and get the ammo displaying 0000, then the CSWeapon.xml is corrupt. This could mean that the unit.dat is corrupt, which will flow on to the CSWeapon.xml being corrupt. Try a reinstall first, if that doesn't fix it, then do this:

Check-in to Your Profile > Select product > Extended Download Service > Get New Links and download the installer again and install from the new one.

Mark