I'm getting a black screen when I try using the GUI. See this screenshot: http://i.imgur.com/LP9DylN.png. How do I fix this?

@DarkBeast thanks for clearing things up. The black screen is due to you connecting to the wrong port on your server. I'm guessing that you are entering port 25565 in the GUI dialog instead of the RemoteBukkit port (configurable, by default is 25564).

The RemoteBukkit plugin uses a different port than the server itself to accept connections. It must do this, as there is no way to share the port with the server without serious craftbukkit hacks. It is this port which you must connect to, not the actual server port.

If you try to connect to the server port, the server and the client will be waiting for messages from eachother, entering a deadlock. Thus, the screen will be completley blank as the GUI waits for the login credentials to be validated. I suppose I should add a timeout feature to the GUI so that users who connect to the wrong port can find out what they have done wrong.

PS: I've clarified your question so that it is more general and can help more users.

PPS: If this doesn't solve your problem, please create another ticket. I will be more than happy to help follow up the issue.