I suspect there is some problem with your connection.
Make sure you have latest Thunder version - update it from Visual Studio Tools >> Extensions and Updates and after update re-install Thunder in your Sitefinity project.
You may also try to create a different admin user for thunder.
Let me know if you need additional directions/assistance.
Regarding the other question for Dynamic Items Field Control Selector - there is a Sitefinity Documentation covering this topic.
I just wanted to let you know that we are looking into this issue and we will post an update here once we have resolved and what was causing it.