Please note that official support for this menu version has now ceased. There are still plenty of users, though, and the forum is still running. Some of our long-time users may be able to help you out.
I've just checked to see if the gap is transparent. I had the idea of putting a table under it with the correct colour. To test this, I've changed the background colour of the page to green, but the gap still appears to be white.
That's the thing. The example you download has the problem. The one on their site doesn't. I'm talking about the one you download as a zip file and run from your hard disk.
Unfortunately I'm not sure what to tell you about the problem. The menu code sent down is the most current. Actually the download sample doesn't even render properly in my browser (latest IE6/XP) - it's sitting on top of some text - but the site page is OK. My guess is there's something in the way it's generating the code. Not much help, I know.