Help me w/ Dreamcast!!! :(

I have one mdf file (Sonic Adventure 2), one mds file (Crazy Taxi 2), and one cdi file (Unreal Tournament) for the Sega Dreamcast emulator, nullDC. The problem is, only one of them works, which is Unreal tournament. I've already tried Chankast and it hasn't worked what-so-ever. Can somebody help me? And is there any way to run these on an actual Dreamcast? ??? ??? ???

mdf and mds are alcohol 120% formats, you may have to mount them onto a virtual drive. Also, to my knowledge, DC emulation never got very far.

:( According to Alcohol 120%, mdf is an unrecognized image file.

thats probably because it needs both .mdf and .mds files for one image. It's somewhat like .cue and .bin