inserting unique, Openx ad codes into category pages

Hi — I am very inexperienced when it comes to coding; most of my site was designed by someone else.

What I would like to do now is place ad serving codes on the category pages, i.e. one code targeted to pages in Category A, and a different code targeted to pages in Category B.

I saw an earlier/ resolved post that suggested creating a category description for each category from the admin panel, which I totally understand: my question is, what do I do then?

If someone could please provide the file I would be updating, where in the file I would be making the change, and then specifically what the code would be to accommodate the ad codes, I would be SO grateful!