In Netbeans 7, Ruby support was dropped:
Although our Ruby support has historically been well received, based on existing low usage trends we are
I realize this answer does not provide a technical solution to the question, but it is an answer of sorts. If you would like to see Oracle include official Ruby support in Netbeans again, please consider signing the following petition:
http://www.ipetitions.com/petition/re-include-ruby-support-in-netbeans/