By default, Mac OS X hides all files that start with a "." (period). This is the normal behavior of Linux/Unix. You may sometimes wish to view all hidden files within a folder.
Open a terminal window (Applications -> Utilities -> Terminal), and type/paste this command:
defaults write com.apple.Finder AppleShowAllFiles YES
Then, hold the Option/Alt key and right-click the Finder icon on the dock and click Relaunch.
To reverse it, type/paste this in a terminal window:
defaults write com.apple.Finder AppleShowAllFiles NO
And then Relaunch Finder again.