On the hands on guide listed in the useful links thread it recommends windows to keep open. One of which is the damage deck window. How do you open the damage deck window??

Hmmm. I wouldn't worry about it. I never leave it open.

But its just the button with a picture of damage deck art at the top bar

You just need to open it once before starting a game, choose which damage deck to use, that will spawn a deck into your player screen.

Sounds like it might be a bit out dated. The damage deck has been moved from the window to your Player Hand window (where you keep your pilot cards, upgrades, and dials). There's a damage deck button in the main window (above Chat), this pops open a window where you pick which deck you're using.

It used to be that players shared a damage deck, but that was a long time ago, if I recall correctly. In those days, it certainly was useful to keep that window open, but now that it spawns in your player window, that advice is obsolete.