Netbeans IDE 7.2 doesn't have Ruby development plugins preinstalled in it.
In order to enable the Ruby plugins follow the instructions:
1) Go to Netbeans Plugin Manager
2) Go to Settings tab and add the link "https://blogs.oracle.com/geertjan/resource/nb-72-community-ruby.xml" as one of the update centers.
3) Once refreshed , It will appear in the list provided in "available plugins" tab
4) Select Ruby On Rails and install it.
That's it. Ruby can now be easily developed using Netbeans IDE :)