I have to switch between Roman and Cyrillic alphabets all the time so I feel your pain, my friend. Fortunately Mac OS X has full Unicode support and switching between keyboard inputs is very easy.
Go to the "International" pane in System Preferences. Under the "Input Menu", check the Arabic inputs and you should be good to go. Check "show input menu in menu bar" if you want to switch easily between QWERTY and Arabic keyboard layouts.