By default, the Mac operating system (OS X) makes hidden files and certain system directories invisible when using file open and save dialog boxes. This is (likely) to make it harder for inexperienced users to overwrite something critical to the system’s performance. However, if you know what you’re doing and need to, for instance, open a PDF manual in a subdirectory of
/usr/local/src, this feature can be a hassle.
With a file open or save dialog open, press the command key (⌘), the shift key, and the period key simultaneously to show hidden files and system directories. Pressing the same key combo subsequently will toggle the view on and off.