Microsoft Office 2011 right-to-left (Arabic) doesn't work
If you want type RTL in Mac OS X I suggest you NeoOffice (it's not 100% like MS Word but it can done most of MS Word Job) or you can use OpenOffice and iWork.

If you want use MS Word 2011 (not earlier version) you can use some trick to solve your problem. make one document in RTL language in Windows MS Word (in your case you must use Arabic) then bring it to mac and open the document and delete the text and save the document as a MS Word Template (your desired name), then each time you need to write text open your template in MS Word 2011 (when you want open the document you have ability to open the desire template).

But you must consider that this solution is not bug free, if you want use RTL and LTR at the same time you will encounter some bugs.