Give Dowd a vial of Ambrosia during the cemetery level. There is only one vial you can obtain before meeting with Dowd, and it can be found in the previous level, in a lab on the PRCS Wall Cloud super freighter upper decks (the area with the sailors and the captain's quarters). Pick it up and keep it in your inventory until you meet with Dowd.
Tip: Make sure you pick up the green vial, not the red one.