I'd like some help with the Mann vs. Machine controls. Thanks to Studoku I now know how to activate my Shield as a Medic, but how do I activate my power-up canteen for the extra Ubercharge?
The canteen is equipped in the action slot, the same slot used for Spellbooks and the Dueling Mini-game.
Then you should be able to use the canteen once it is filled by using the H key. Which is the action key.
Pending your button layout it could also be the G key.