Maybe it is a javascript error.
Please goto to uddeIM backend, system tab. There you can specify how uddeIM handles MooTools. Usually "auto" should be fine but you can also try "do not load MooTools, use 1.x". The problem with Joomla 1.5 is that it brings it own MooTools, so it might have been already loaded by other tools.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer