a-ha! I found your forum. I think I can help you with this now.
First, your IFRAME tag should look like this:
That looks almost exactly like yours, but you'll see I've added a forward slash in front of the URL to indicate that the "forum" directory is in the web root. EDIT I also took out "display: none" from the style attribute, and removed the trailing forward slash!
If you make those two changes and you still have trouble, let me know!