Hi,
There's a missing section which is needed in order for the addin to be loaded (the "name" attribute's value is specific to my addin of course):
<Manifest>
<Identity name="HelloSDAddIn.UserControl1"></Identity>
</Manifest>
It took me almost a day's work + debug #develop in order to understand what's wrong so I guess it is worth while adding this section as a default.
Thanks,
Yoav Sadeh