It's only rated for 5 lbs and I've never seen the side brackets. Didn't know they made them.
Link?
They're supposed to be part of the package. They bolt to the tab that extends down from the rack and support it by connecting to the main bracket. I wouldn't trust the rack to hold even the rated 5 lbs. without it.
I'll get a pic of mine tomorrow if no one else posts one.
EDIT: Here's a couple of pics I found that shows it.