as far I can tell the 'C.harddisk/EXPRESS' is the install location. This should be mounted as the C. Im confused why you keep trying to mount it as D.
you want to mount the iso image as the D, try using the full path you gave, so something like: /Users/mm88/DOS Games/Express 1 1.boxer/EXPRESS.iso
My general approach to my old dos cdrom games is to make the iso image of CDROM. I have a general location for all my dos games, which I mount as the C drive. I mount my new iso file as the D. I can then install and do everthing like it would work on a real dos system.
I also suggest you read over the dosbox readme and other help material online and build up some knowledge about dosbox in general.