i need to add the eloqua js script to an existing landing page

There's a few things you could do... the easiest is go into the landing page's template and add it there. You can also do it on an individual landing page level by dropping it into the custom head section (if the code needs to be there), or you might be able to drop the script into an HTML object via the landing page editor.

Your best bet is the first method, as it'll give you the most control.