http://www.dosbox.com/wiki/Usage
Usage
name
If name is a directory, DOSBox will mount the specified directory as the C drive.
If name is an executable, DOSBox will mount the directory of name as the C drive, and start executing name.
add the extension of the executable also – and make sure the path is right etc.
note you should “apt-get remove dosbox” and use the retropie packaged version for best performance – /opt/retropie/emulators/dosbox/bin/dosbox
(get the latest retropie 2.4 beta image or update binaries on your current one if it isn’t there)