More than the entry, if you're missing the actual initrd for your new kernel, that's what you got to solve. Look into /boot to see if you got a initrd-2.4.18-26, if so, just add the correct entry to lilo.conf [...and run /sbin/lilo].
If not, find out what kernel pkgs up2date actually installed.
rpm -qa | grep kernel
as you'll most likely need the kernel-sources to create a new initrd [...maybe the headers too???].
Anyway, see man mkinitrd and mkinitrd --help, iirc it was quite easy to figure but it's been a while since I've had to mess with that and the distros I run now,don't use it.