How about an image of what you want. Otherwise the general answer is to edit a template and add in your ad code. If you were more specific, then we could be more specific.
Hmm, when I do this I only see one space for the HTML for a leaderboard. Do I put both ad codes into that one html box?
Also, I have some approval ad code and I need to add to a header file so I can begin selling ads (not through adsense). Where would this header file live?