FOLDER/templates/subSilver or the theme you currently have...
And then edit the file "overall_header.tpl"....
Then you put the code on the very top of that file...
You save it and you have it done
And if you want to put the ads under your logo and navigation links, put the code on the bottom of the file, or just after navigation links (like {MEMBERLIST} or {PROFILE} and some html code).
Oh.. I don't really know...
I told you how to put the banner on the top of the site, specialy for the corporate plan of free hosting...
I think you'll have to investigate the files a little