I played around with dd and found out that dd-ing the live CD to a USB stick works fine, so I suspect operator error.

I set up the computer BIOS to boot off of USB and used an Ubuntu 12.04 Live CD. Popped the Live CD into the drive. My drive is /dev/sr0 and the USB stick is /dev/sdb.

I entered this code in terminal:

sudo dd if=/dev/sr0 of=/dev/sdb
Assuming you were using the correct Slackware CDROM, you may just have gotten the device ID of the USB stick or the CDROM drive wrong or something like that; hard to say.