Here's the page with the video.
I will point out that the video shows casting on w/o the slip knot. I much prefer this method to using a slip knot and this may solve your problem if you learn how to do it. It's called long tail cast on. There is a video of it here in the video section.
If you need to use a slip knot for now just knit that slip knot as a stitch and keep going.