If you have the "Sabotaging Jimmy's Operation" make sure you get all 4 sabotage quests
- Drugged Chicken
- Cheat Detector 1
- Cheat Detector 2
- Cheat Detector 3
"Caught in the Act" talk to the Boy Naoto on the Airship to advance to the Sabotage the engines part. If you couldn't advance past this part before, please try it again.
If you are trying to fire the cannon, please be patient and keep trying to light it (click) it will eventually fire.