I am a newbie, please help...some questions about using boot disc and self boot games

Hi, I want to know that do I need to modify my Dreamcast in order to use boot disc and self-booting games? And one more question, the .cdi images do not seem to work with Discjuggler...why people keep saying to burn them with that prog? When I load a .cdi image, the "start" button of Discjuggler is still gray (disable), can someone please tell me why? My cdr is in the burner...email me if you can, a reply here is fine also, thanks a million!