Ok, look in /var see if there is anything in there. If not, then let's make the directory it's asking for:
mkdir -p /var/log/mount/slackware
Then, figure out which device your cdrom is:
Primary Master = /dev/hda
Primary Slave = /dev/hdb
Secondary Master = /dev/hdc
Secondary Slave = /dev/hdd
After that, using the command from fancypiper, go ahead and mount the device:
mount -t iso9660 /dev/hdx /var/log/mount/slackware
replacing /dev/hdx with the proper device from my table above
Cool