Bevore you can use the files located on your own filesystem,
You have to mount the directory containing the files.
For Windows:
Mount c c
dosprog will create a C drive in dosbox with c

doprog as contents.
For other platforms:
Mount c /home/user/dosprog will do the same.
When the mount has succesfully completed you can type
c: to go to your freshly mounted C-drive. Typing
dir there will show its contents.
Cd will allow you to enter a directory (recognised by the [] in a directory listing).
You can run programs/files which end with .
exe .bat and .
com.
...