var how_many_ads = 3;
If you have 2 ads, change that to 2, and so on.
url="http://LINK1"; alt="Alt Text"; banner="BUTTON1"; width="88"; height="31";
Where it says LINK1, you obviously put in what the button links to. Alt Text specifies what words come up if you hover over the button; and the BUTTON1 should be replaced with the URL to the button image. You can also customize width and height.