ZamFear: The download contains a batch file intended to launch DN3D with the command line parameters needed to start the custom level.
Probably doesn't work because you have to run DN3D through DOSBox. The batch file is written for an old system where DN3D would just work.
You should be able to get it running by editing the dosbox.conf file. Near the bottom, you should find something like this:
[autoexec]
# Lines in this section will be run at startup.
@ECHO OFF
mount C "."
c:
cls
duke3d.exe
exit
Easiest is probably just to change "duke3d.exe" to "redrum.bat" (change it back when finished, of course).
I think you can also launch custom levels using the setup program, but I'm not sure about that.
Or you can use eDuke32 which has a level / mod selector.
