After more research: 1. You must be using SCIM which has an Arabic keyboard layout for western users. On this site you will find character map:
http://michsan.blogspot.com/2008/06/...with-scim.html
However, if you want traditional Arabic keyboard as in Windows, then don't use SCIM. Go to System-> Preferences-> Keyboard. Select 'Layouts' tab (second tab). Click on 'Add' button. Click on layouts drop-down menu. Chose Arabic. Click Add. Now you are ready to go. Click on 'Layout Options' button to add more options if needed, e.g. by default you will switch between Arabic and English layout by pressing both Alt keys. You can change that and many other things you like. Now you can type Arabic the same way as you used to in Windows.
PS: If you do not see Arabic in layout menu, go to Systems-> Administration-> Language Support and install Arabic language support.
Good Luck!