Hi,
It looks like Arabic proofing tool for Office 2013 is not available for download directly. To get Arabic proofing tool, alternatively, you can install a localized version of Office 2013 which includes Arabic as a Companion proofing languages.
Each language version of Office 2013 includes proofing tools for a set of companion languages. For example, when you deploy the English version of an Office 2013 product, users receive proofing tools for both Spanish and French in addition to English.
For a complete list of Companion proofing languages for Office 2013, please refer to:
http://technet.microsoft.com/en-us/library/3f4de10b-757a-4ce5-b9b7-1baafeb4753e(v=office.15)#BKMK_Identify
Regards,
Ethan Hua
TechNet Community Support