Keyboard Shortcut To Switch Between Opened Windows
Solution 1:
Cmd+` will allow you to switch between the open windows of the current application. Very useful.
Solution 2:
By default (on a clean install of Mountain Lion), there is a keyboard shortcut "Move focus to active or next window", ^F4, which worked for activating the BetterSnapTool preference window (which is the same deal, activating a window in an app that doesn't have a dock icon).