It sounds to me like when installing DSL, you overwrote your existing GRUB configuration. What needs to be done now is to edit the GRUB config file to include an option for your other distro.
By the way, you didn't screw up, since you would have had to perform these steps either way: if you hadn't overwritten the GRUB configuration, you'd be wondering why DSL didn't show up in the menu, and I'd be telling you to perform these same steps
Boot into DSL, and (as root) edit the file under /boot/grub/menu.lst. You should be able to essentially copy the whole block for DSL to create a new entry for Knoppix. The only real difference should be your partition identifier numbers (keep in mind GRUB counts from 0).
There's one caveat with this setup: Your GRUB configuration now resides under DSL's /boot/grub/ directory, and from the sounds of it, Knoppix is your main distro. You can of course boot into Knoppix, edit the same file there, and reinstall GRUB so that it uses the Knoppix configuration. This is only an issue if you are considering removing DSL from your system, in which case you would delete the configuration GRUB is currently using, resulting in (if I recall correctly) a GRUB error 17. Cheers, best of luck,