You can install eclipse plugin in Rhostudio just like you'd do in eclipse : click "help" then "install new software" and then you have to enter the url of the plugin you want to install.
One convenient way is to first install the market place, then you can install any plugin you want from the market place.
To install the market place on Rhostudio 2.2 :
help->install new software->clic "add"->enter "eclipse indigo" as name and "http://download.eclipse.org/releases/indigo" as url
wait untill the list is filled in the bottom window,
select "general purpose tools" and then select "marcketplace client"
Thanks Nicolas. That helped a lot.