Vaadin plugin for Rational

I am trying to install Vaadin plugin for IBM Rational Application Developer for WebSphere (v9). I did install the Eclipse marketplace plugin and then found “Vaadin Plugin for Eclipse 2.3.5.20150923”, but the installation does not go past “Confirm” button.

There is an error saying: "
No repository found at http://vaadin.com/eclipse/.
".

How to do this?

arto