Take a look at the way the page is layed out currently with the divs floating left and right. Now, go google something like "layout three columns css" and you'll find a page such as this -
http://bluerobot.com/web/layouts/layout3.html . Take a look at the css for those three columns and then you need to incorporate that css into your design. As Hanson said, it isn't quite as easy as you probably want it to be. It's not impossible, but it isn't a matter of just changing a couple of lines and having it work.
(The easy way would be to just put a table around it and have a new column for your stuff and the second column for what is currently in the page. Putting a table there, though, kinda ruins what they were doing by having it all done via divs. I think that's an ugly solution, but it is a solution.)