As of yet (2016-07), WAS 9.0 is not supported by Rational Application Developer 9.5.
Instead, download and install Eclipse Neon, then install in it the "WebSphere Developer Tools for Eclipse Neon" from:
Unzip the latter to folder "WAS9plugin" (my file was called libertytools-update-site_18.104.22.168.v20160602_2252.zip) and in Eclipse Neon do menu > Install New Software > Add > Local > browse and select folder "WAS9plugin" > OK button > in the Available software dialog check the item that appears called "WebSphere Application Server Liberty Tools for Eclipse 22.214.171.124" > click Finish.
Once installed and Eclipse has rebooted, menu Help > Install WebSphere Software > scroll to the bottom of the list > select to Install "WebSphere Application Server V9.0 Tools" > Finish button.
You will then be able to add the Servers view to the Eclipse workbench, then right-click Servers > select New > WebSphere Application Server 9.0