There are a couple of possible reasons.

First the viewlet source is not pointed at the proper repository.  After clicking the Add Widget, there is a "Select Catalog:" make sure it is the one you wish to use.  The context of the dashboard ( project, personal) has influence on the behavior of the initial choice on that "Select Catalog" menu.  

Second, the report resource is not installed in the Project Area.   Go to the Report / Report Resources and look for "Micro Release..." or "Micro" anything.   If those aren't in the project, there's an icon on that page ( upper right of the list, looks like part of a page and flashlight ) when hovered will say "Deploy new resources".  Click that and locate missing resources.