There are some questions about your lilo.conf.
Did lilo run without any errormessages?
Why did you insert quotationmarks in the line
If Slack11 recognizes your disk as /dev/hda and Slack13 as /dev/sda, I'd suggest to boot Slack11 like Windows (with "other"). Change the lilo.conf in you old Slackware-11 installation from "boot=/dev/hda" to "boot=/dev/hda5". This will write your Slack11 lilo onto the mbr of the partition (not the disk).
Then add Slack11 like Windows with "other" in lilo.conf of the Slackware-13 installation.
other = /dev/sda5
label = Slack-11
don't forget to boot Slack-11 and execute lilo there. You may remove the line "Prompt" in your lilo.conf in Slackware-11, since you'll not have to chose an operatingsystem there.