STEP1> Install

$ rvm install macruby-nightly
$ rm -rf /Library/Frameworks/MacRuby.framework
$ rm /usr/local/bin/mac*
$ rm /usr/local/bin/hotcocoa

Install nightly build version 
$ wget http://www.macruby.org/files/nightlies/macruby_nightly-latest.pkg
$ open macruby_nightly-latest.pkg 
$ cd /Users/jaigouk/Developer/.rvm/rubies/macruby-nightly/bin
$ mate .

STEP2> change_erb_ruby_testrb_irb_rake_etc

Change these lines in each exec files

exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macerb" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macruby" "$@"     
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/mactestrb" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macirb" "$@"        
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macrake" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macrdoc" "$@"
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macri" "$@"
exec "/Library/Frameworks/MacRuby.framework/Versions/0.6/usr/bin/macgem" "$@"

to these =>

exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macerb" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macruby" "$@"     
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/mactestrb" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macirb" "$@"        
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macrake" "$@"   
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macrdoc" "$@"
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macri" "$@"
exec "/Library/Frameworks/MacRuby.framework/Versions/0.7/usr/bin/macgem" "$@"

STEP3> install rspec

$gem install rspec --pre