When you download dban, it should be an ISO file, not a RAR. If you have WinRAR installed, it probably changed it's file association to a RAR, but it is still an ISO. Don't use WinRar to extract it.
In ImgBurn, you select the first option "Write image file to disc". At the top of the window you must browse to the .ISO.
If it is not allowing you to start the burn process because of "Device Not Ready" you either don't have a new blank disc inserted, something else is trying to use the disc (windows is wanting to burn something to it) or you have a problem with your drive.