This will depend on your skin and whether there is a pane where you want the banner to go. You'll also need to know how to find your skin files and edit them.

Look for the skin ASCX file that your site is using, find the place where you want the banner to go and insert the banner code directly into the skin code.

Or, if there is a Pane where the code is to go, just insert a Text/HTML module and paste the banner code in Source view or switch it to Text-only then HTML view and then paste it in.

Or, if there is no Pane there, you can create your own in the ASCX by examining how any other pane is coded in the skin. So long as the Pane has its own unique name and id, it will become available on the DNN control Panel and you can then insert a Text/HTML module for your banner code as above.

Hope that helps,
Rob