Notes on a clean install to Ubuntu with RoR and MongoDB. Has you do all the configuration yourself though, no use of the plugin. This is a last resort.
Another useful doc here. It is the MongoDB doc on installing on Ubuntu.
Did you restart RubyMine after installing the plugin? Log out / Restart from your current session on desktop?
When you installed MongoDB, did you use the default repo that came with Ubuntu or did you use the link mentioned above to install?
-Is the Mongo server actually running?
--Go to terminal and try sudo service mongodb restart
---Try accessing the URI for administration. See here