As far as I can tell (searched the internet far and wide), Photoshop is completely missing a menu item in the 'keyboard shortcuts' -menu to change the shortcut for "activate a mask".
So, normally, to activate a mask on the currently selected layer, you press Ctrl \ (Mac: Command \). This is the same as clicking on the layer mask thumbnail. HOWEVER, there seems to be no way to change this shortcut in 'keyboard shortcuts' -menu (the item is missing).
Why would you want to, since there's already a shortcut set??? Well, if like me, you're using a foreign keyboard, then the backlash won't function here. So the only way I could use the original shortcut would be to change my keyboard layout completely to a US keyboard.
Please Adobe, add a menu item where this shortcut (Ctrl \) can be altered.