Selecting more than one non adjacent categories

I would like to have komento displaying on more than one category, however, the categories I want komento to show are not next to each other. How do I select two categories that are not above/below each other?
I'm sure this is a real easy thing to do, I just don't get it...

This is exactly what I mean:

.| Category 1

.| Category 2

.| Category 3

I would like komento to be on category one and three, is there a way to select these without selecting category 2?