From the command you've listed, it appears you are running the md5sum program against the actual md5sum verification file (eg, the 1K text file containing the md5sum) rather than the actual Slackware ISO image (eg, the 3.8G ISO file). The actual name of the ISO image should be something like slackware-11.0-install-dvd.iso (no trailing .md5 extension) and the command should be something like
The output will be a long series of letters and numbers need to match exactly to the contents of the slackware-11.0-install-dvd.iso.md5 text file. If they don't, then your ISO is corrupt and you'll need to download it again.
I recommend burning at a slower speed than your burner's top speed, and to always use quality, name brand media. Burning at or close to the DVD's top speed and using random no-name blanks almost always leads to coasters. Good luck with it