I've just installed MacRuby (MacRuby version 0.4 (ruby 1.9.1) [universal-darwin9.5, x86_64]) via the package I downloaded from http://www.macruby.org and I dropped straight into macirb to start on the tutorial (http://www.macruby.org/documentation/tutorial.html). I ran framwork 'Cocoa' and uh oh.. a RuntimeError popped up.
I've seen another thread on this list from January with the same issue but there wasn't really any fix detailed. I've posted my console output of lots of different commands at https://gist.github.com/359195517cc2364b82b8.
I'm running 10.5.8 on a MacBook Pro 2.4Ghz Intel Core 2 Duo. Hard disk format is Mac OS Extended (Journaled).