When ever I write something in Arabic or Persian in Adobe Illustrator CS5.1 the word do not appear on the screen? But I can see the word which I have written in layer names.
It is possible that the font you're using is not supported by Illustrator. I would suggest trying another Arabic font. Reference: "Arabic and Hebrew type"
Legacy font support
Fonts that have been traditionally used (for example, AXT fonts) can continue to be used in this release of the software. However, it is recommended that newer Open Type fonts be used for text-based elements. Missing Glyph Protection (Edit > Preferences > Advanced Type) is enabled by default. Text is handled automatically, where glyphs are not available in the font you are using.
If you still have an issue please edit your question with more information.