best way would be to add teh following line at the end of dosbox.conf (/usr/share/dosbox/dosbox.conf):

mount d /media/mmc1/

mmc1 is the external card, mmc2 the internal one, so change that apropriately if needed.

You also can type it directly into DosBox, but with the line in dosbox.conf it will be mounted automagically each time you start up DosBox.