One tester responds with this tweet:
From some beta testers I have spoken with, it appears that if one is working from a document created in another application that does have right-to-left Unicode that you can have more success.
Yes I did, when I edit an Arabic file it works perfect, but it doesn't if I create a new doc!
And another stated this way:
About Arabic support in MS office 2011 Beta 5 for Mac: 1-Word:YES if you open an Arabic doc 2-Excel:NO AT ALL 3-Power Point:YES PERFECT