RubyMine 1.0 checks that interpreter executable file name starts with "ruby" prefix, otherwise RM will show error message "The selected file is not a valid home for RubySDK" (RM | File | Settings | Ruby SDK and Gems | Add Sdk..). So
* for MacRuby Ruby SDK you should create symbolic link "rubymac" to "/usr/local/bin/macruby"
* for IronRuby have a look at instructions at http://flanders.co.nz/2009/05/02/using-ironruby-with-rubymine/ (thanks to Ivan Porto Carrero)
P.S: Don't forget that RubyMine 1.0 supports only Ruby 1.8.x language version so it will highlight new features of 1.9.x syntax with errors.