Hi Abdel,
I’m sorry to tell you that Arabic is not supported in Office for Mac at present. We have a specialized forum supporting Office desktop forum, it is suggested you post a thread there to ask if any workaround is available: http://answers.microsoft.com/en-us/mac/forum/macoffice2011?tab=Threads
Besides, the Office Online applications support Arabic language. You can access those applications via the Office 365 Portal.
Best Regards,
Eric Yang