The NetBeans newsletter headlined with Ruby announcement so I went to Ruby/Rails NetBeans Plugin page and saw it has been verified for 7.0. This means I can click "reload catalog" here and then I can install the plugin, because one of predefined plugin centers in NetBeans is for verified community plugins:
So no need to download plugin from plugin page. Just click the "reload catalog" button and then I can install it immediately into NetBeans and use it like before... but this time in NetBeans 7. Thanks Tom Enebo and Ruby community.