I have a mac and i can't open pkg and mpkg files plz help!?

".pkg is a file format used by Apple, Inc. on its Macintosh line of computers, and the iPhone. It's also used by Sony's PS3. Not much is known about the format, although it is known that the files are very compressed. The format is opened by Apple's "Installer" app, which reads the .pkg file and installs the software it contains. The .pkg file format allows easy installation of software by end users, and is commonly used by both Apple and third-party developers."