1. Execute defaults write com.apple.finder AppleShowAllFiles YES
2. Open the 'Force Quit' menu from the 'Apple' menu, or with the 'meta+option+escape' key combination (also known as 'command+option+escape').
3. Locate 'Finder' in the list, click on it to select it, and then click the 'Relaunch' button.

Now hidden files will be shown. I assume the same procedure with NO instead of YES will undo it.