The first thing, before you connect any board, is to allow the connection to websockets from the SSL secured IDE. You can do this by clicking the settings or lock icon in the URL address bar and then click the Site settings button.
Now you have to allow Insecure content, so that the SerialLink application, which does not uses the SSL and is running locally can connect to the CodeSkool IDE, which is a SSL secured website and running on HTTPS connection.
Once you have allowed the insecure content, you need to refresh the CodeSkool IDE, by pressing the Ctrl + F5 keys.