The installer ships lilo and asks you if you want to use it. If yes it installs a boot sector at end of installation (with lilo, or elilo in case of EFI). If that didn't work for some reason you can use the Slackware DVD to boot into your installed system, then update /etc/lilo.conf as you see fit and run lilo, or use the "liloconfig" utility to help you do that.
So boot of the Slackware DVD and follow instructions in the first screen to get into your already installed system instead of re-installing, the re-install lilo.
If you are familiar with that you can also run the installer up to the point where it tells you to login as root, then mount the / partition of your installed system somewhere, bind mount proc, sys and dev then chroot to the installed system and proceed as in the first method.
Last edited by Didier Spaier; 04-11-2014 at 04:51 PM.