[The Fifth Horseman]

I recall Discworld II is a two-disc game. You'll have to do it more or less like this:

IMGMOUNT D "C:/path/to/the/disc/images/CD1.iso" "C:/path/to/the/disc/images/CD2.iso" -t iso

Where C:/path/to/the/disc/images/ is replaced by the path you have the disc images located at and CD1.iso and CD2.iso are replaced with the names and exptensions of the actual images.
When DOSBox is running, you can now change between the two mounted disc images by pressing CTRL+F4.
This is best entered into your DOSBox configuration file's autoexec section so that the images are mounted automatically when the emulator starts.