Version of InDesign? Operating system?
indesign CC 2015 , Macbook pro 10.11.3
In case you don't have a ME version you can type in Arabic (Hebrew as well) with this little trick:
-click in your block with the Type tool (until cursor blinks)
-choose an arabic Font
-Fill with Placeholder text while maintaining the Command/Contol key, choose Arabic
-remove the generated rubbish text and type your own text instead (or copy/paste), this will force the block to stick to Arabic.