When you start OneSwarm then firefox starts automatically, and then look carefully you see your community servers load on the right side of the search field.
If you click on it while it is trying to load a community server (which it always does in the beginning) then you get a popup telling you what is going on.
NAT settings have nothing to do with community servers, and your problem could be that your Community Server is down. Many of my community servers sometime never respond for a whole week. Sometimes they do respond, but no connection to any of the new users.
An easy way of checking that your community server works is to press the icon "Friends". After loading that page you can click once at the column "source". Then the list gets sorted so you can see users for every community server. Next to it you have a "Date added" showing WHEN the user was added. You can also see if you ever have had a connection to that friend.