First remember that Mac paths differ somehow from those UNIX paths, wich are the ones used by DosBox (and virtually by any program, although you will not see it).
The actual path of almost everything in your computer is usually this one:
/Volumes/Macintosh HD/...whatever
in UNIX, spaces are substituted by backslashes, that is:
/Volumes/Macintosh\ HD/...
Or you may use quotes ("/Volumes/Macintosh HD/...")
Again, I advice you use a graphical front-end. Try Petit dosbox at:
http://web.jet.es/guilly/slouc/software_petitdosbox.html