first thing is to be sure your html is enabled on the forum. once that is enabled then you can pretty much use any html codes anywhere.
admin panel>general>messages and emails. scroll down to allow html click yes and hit save.
now go to your forums home page index. go to profile.profile preferences and scroll down to always allow html. select yes and click save. now your html is enabled for the whole forum and the question would be is where do you want your adds?
I figured out how to post HTML under the page heading, but is there a way to put the HTML so it appears on the sides of the forum boxes? I was thinking it may have something to do with "creating a new HTML page" but without knowing the script to use the forum itself I don't know how exactly to use "creating a new HTML page," by using this where does the HTML take effect at?