Okay, try this
QUOTE
Players
- Humans
Conditions
- Get new weapon
Actions
- Move weapon to Dropship Load
- Run AI Script "enter transport"
- Set Deaths: Add 1 for unused Unit
QUOTE
Players
- Humans
Conditions
- Current Player has suffered at least 1 death of Unused Unit
- Bring at least 1 men to Dropship Load 2 (ground only location)
Actions
- Display Text "You have too many weapons"
- Other stuff
- Preserve Trigger
That should work, if it doesn't, try adding a wait between the AI script Enter Transport and adding the death counter.