paint.net for Mac is not going to happen.
Quoted from FAQ:
(1) Will it ever be ported to Mac OS, Linux, or any other operating system?
(2) What about Mono support? Wouldn't that be really easy?
We will not be doing any work to directly support Mac OS, Linux, Mono, or any other platform. We are doing this in order to focus on the best quality and support for the platform that we develop on: Windows with .NET. Also, we simply do not have the resources or expertise to do any of this work.