The default installation of Mac OS X come bundled with native Gurmukhi support and enabling it is very easy.
Start by going to the Apple () > System Preferences

Click on the "International" preference item.

Click on the "Input Menu" tab.

Scroll down and check "Gurmukhi" and "Gurmukhi - QWERTY". Ensure "Show input menu in menu bar" is also checked.

You can now switch keyboards layouts using the menu bar or by using the shortcuts specified above.

The "Gurmukhi" keyboard layout is based on INSCRIPT keyboard layout and is similar to the Jhelum (http://code.google.com/p/gurmukhi/downloads/list) which can be installed by placing the file in /Library/Keyboard Layouts and enabling it in the International preference pane.

The "Gurmukhi - QWERTY" keyboard layout is more usable for those people who are used to using an English QWERTY based keyboard. All letters are placed in their phonetic latin equivalent. Note that some characters may require you hold the [Option] key or [Option-Shift] key combination. For example, to type ਡ you would type [Option - D] because there are four characters that correspond with D in Gurmukhi (ਦ, ਧ, ਡ, ਢ).

You can now natively use unicode Gurmukhi throughout Mac OS X, including sending emails, chatting, etc. Please note that you will not be able to use the fonts on Project Satluj's website because Apple does not fully support OpenType. This means you are restricted to Gurmukhi MT font that comes bundled with Mac OS X.
