Open the terminal app from “Applcations > Utilities”.
To show all files enter:
defaults write com.apple.finder AppleShowAllFiles 1 && killall Finder
To hide hidden files again
defaults write com.apple.finder AppleShowAllFiles 0 && killall Finder
1 = show all files
0 = do not show all files
In OS X Mountain Lion / Mavericks, you can no longer open hidden folders normally (by double-clicking).
You can however, open hidden folders by right-clicking on them, and clicking “Open”.
On previous versions of mac os:
To show all files enter:
defaults write com.apple.finder AppleShowAllFiles TRUE && killall Finder
To hide hidden files again
defaults write com.apple.finder AppleShowAllFiles FALSE && killall Finder