You are correct Steve. It appears there is another bug that User Interfaces can't have absolutely positioned elements added after the page has rendered.
It is another bug in Dreamweaver, not really ours, but one that we can probably work around. We will need a couple of weeks though to update all of our UI's.
I got it working with the Dreamweaver extension manager... no need for the DMX one, but it looks nice if you want to use it.
I just had to uninstall and reinstall the Extension Manager and then uninstall and reinstall all of the extensions (completely removing the old ones first). I'm at the same point as you now and I'm working on a solution.