That is odd - I know there have been some changes made to UiPath.Core and UiPath.Famework Activities - do you have the option for the latter?
Failing that I have attached the UiPath.Core.Activities file from my install which you try adding to your C:\Program Files (x86)\UiPath Studio\Packages or creating a custom import location by right-clicking on the left of the image you have sent and selecting Configure Sources.
The central repository is the Available section in that section - I believe there is a problem with the latest version or at least some misunderstanding about where Activities are - I’ve asked UiPath to comment as another user had similar issues.