Ok, I got it.
If you double-click on a server in the Find Servers list, you never have the Download Addons tab! I tried with Kellys Heroes and it isn't there. But if you add the server to your favorites and double-click from there you always get the Download Addons tab. In this case when you click on the Check for Addons button you sometimes get a not found error, probably because there's no repository.
Either something was changed in the latest Yoma's version or this has something to do with .NET versions, which seems odd.