I guess you want to change the direction in the portal output as well as in the Kajona administration?
For the portal output you just need to modify your CSS files, that's pretty easy.
For the administration you need to modify some files since Kajona doesn't support this out of the box right now.
I guess you need to add the attribute dir="rtl" to all input fields and maybe use the CSS rule "direction:RTL;" - but I never tried it before.
You'll find all the HTML code in the admin skin: admin/skins/kajona_v3/elements.tpl
There's also the CSS file: styles.css
Furthermore you need to set some configs for used tools like CKEditor. It supports RTL, just have a look here and choose the Arabic language: http://ckeditor.com/demo
Let me know if you need more help with it.