I did a quick run though last night after receiving the beta email, and it all went well.
I installed in in DW cs 5.5, alongside a bunch of other WA extensions.
I wasnt exactly sure what the extension was supposed to do, so like Daryl, I applied it to a form with no spry validation, and got the warning message.
So, it looks like this extension just extends the functionality of the spry validation error displays. That is great, since one of the things I hated about spry validation was that it was a pain to configure the error styling.
The only thing I thought would be handy would be a preview of how each theme looked without having to click on it. Perhaps even just a link that opened a popup that showed a list and example of all the themes in one spot, so I could pick the one I liked easier.
Anyway, it looks like a great addition to the default dreamweaver spry validation.