If the connection can't be established it's because of most of the public nodes are under heavy load. It should work when you wait some time in the starting-dialog because the tool tests different nodes until one that is working has been found.
I changed the default setting to the Steemit node today because it's the most reliable. You can clear your browser cache and visit SteemWorld after that or choose the node 'api.steemit.com' in the settings.