Icons for OSX

[kevin at stallibrass.com]

I'm probably doing this backwards but I'm developing with 2.5 on a PC then
building standalones for OSX & PC (gives me a pleasant surprise when I first
view on the mac)
I'm stuck on creating OSX application icons. 
For Windows I'm using Articons and that works fine for the PC side but the
icons are not recognised by Rev when I try to select them for OSX.
I've read about Icon Composer but have no access to it (how do I get it?)
What format are OSX icons?
 
Thanks
Kevin Stallibrass