I'm guessing you're having a problem with the /

Try typing the \ instead. Non-English keyboards have problems with DOSBox.

else switch to keyboard layout US.

or put the mount commands in the [autoexec] section of the configfile(dosbox.conf)

*example: *
[autoexec]
mount c c:\
mount d d:\ -t cdrom
c:
cd kees
superman.exe