You can't use real game discs in the emulators - your PC drive will not read them. For your machine, either Demul or nullDC should run well and give you more compatibility than Chankast.
How do you read them then, dismantle a dreamcast and put the system cd-rom drive as a usb(if so How and were would be the drivers and programs for this) or go online and download them(and no, I not asking for were you download them at)