You can either manually type the mount command everytime you enter dosbox, or edit the dosbox.conf file and you'll see at the very bottom of this file is where you put the commands. (FWIW, I originally put my dosbox.conf file in my '/home/user/' directory. With ArnimS's latest dosbox install that he just put up tonight, the dosbox.conf file is in the '/usr/share/dosbox/' directory). I've got my dos games over on my internal media drive in the 'dos' directory that I created. So I mount that directory with the following command and then use the 'c:' command to go to it:
mount c: /media/mmc2/dos
c:

Then say I've got the keen1 game files copied to the directory 'dos/keen1', Since I'm already in the 'dos' directory from mounting it and changing to it, I just go to the keen1 directory by typing 'cd keen1' and then just type 'keen1' to run the game.

If you need any other help, let me know.