When you get to the end of the row, use the "knit on" method to cast on the additional stitches: pull the yarn through as if to knit, but instead of pulling the stitch off of your needle, put the loop that you just formed back onto the left needle.
Watch Amy's "knitting on" video on this page
and take a good look at how to put the stitch on the left needle. I did it wrong at first because I missed that you had to kind of twist it.
Hope that helps! :D