How can I set Java Platform for JRuby Project?

By default, JRuby will be run with the same Java Platform (JVM or JDK) as the one running NetBeans (which you in turn can control with the --jdkhome flag). You can control which Java Platform is used for JRuby by using the System property jruby.java.home. Edit your etc/netbeans.conf file and add the following -J-Djruby.java.home=directory, e.g:

-J-Djruby.java.home=/System/Library/Frameworks/JavaVM.framework/Versions/1.5/Home

It is currently impossible via NetBeans UI. To be fixed in the future.