Urdu language is written in a way that the letters are joined to each other. But on my Mac OSX 10.7.5 the fonts working fine in Windows are not joining to each other in most applications like MS office. I have tried several Urdu fonts but same problem for all. The most popular font can be downloaded at http://www.4shared.com/file/xOTOiQiW/Jameel-Noori-Nastaleeq.html.
However these fonts are working fine with TextEdit only i.e. TextEdit is joining the letters without a problem.