The register widget icon is located in the Solution Explorer
. The screenshot which you gave is of the Sitefinity Explorer
. You can also check this video
for an example of a widget registration through Thunder.
This video and many others as well as the documentation is now available through Thunder itself. All you need to do is open Thunder Start Page
from the Visual Studio menu -> Telerik
-> Sitefinity Thunder
-> Thunder Start Page
. You can also open it from the Sitefinity Explorer
- it is the last icon which you can see in your screenshot. I would highly recommend checking it out for more information about Thunder. We are currently working on making pretty much all of Thunder's functionality available from there as well.
Please let me know if I can be of further assistance.
All the best,
the Telerik team