Check out this link for an example of how to build iOS apps with it: http://www.thomasdenney.co.uk/blog/2015/1/27/nim-on-ios/

Because it compiles directly to C, I've found it reasonably easy to bring into different environments myself, though I've not attempted iOS yet :)

Edited to add: Also check out https://github.com/yglukhov/nimx