You cannot use Windows fonts to write Arabic in most Mac apps (TextEdit is the exception). The two platforms use different technologies for complex scripts like that.
Some additional fonts that may work can be found at
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=ArabicFonts
http://www.redlers.com/downloadfont.html
In general all iWork apps have bugs that make them unsuitable for RTL scripts for most purposes. Copy/paste from another app may work best. A note that may be useful is at
http://www.cs.technion.ac.il/~shaulm/keynote/