Copy and paste the CD.id file then. The game needs to be installed correctly and it really really wants to be installed in: "C:\DARK"
Dosbox needs to be configured correctly, but it all should work (I don't think DF was one of those games that required the CD in the drive to be played if you copied everything over and properly installed it on your hard drive). Hopefully you're using the real game and not some pirated version that might have screwed things up or be missing a file. Both the demo and the full version work in DOS box.