I do freelance work, but I went ahead and figured this out. Check this in as many different browsers as you can.
In header.php (I think, depending on your theme adn where you added the menu code), in between <ul class="pages"> and <ul id="dropmenu"> is an extra <li> </li>, so take that out but leave the <li>.
Then, right above </div><!-- End navigation -->, add a </ul>, so it looks like
</ul> </li> </ul></div><!-- End navigation -->
instead of
</ul> </li></div><!-- End navigation -->
And in style.css, near line 480, change
.col-555 { width:580px; }
to
.col-555 { width:525px; }