Hi @Anonymous
the "Download bundled libraries" button origins in EAGLE and, to be honest, I am not sure if it is expected to be in the Fusion Library Manager. I will discuss with our dev team about its purpose and the intended action....
But as a quick workaround you could sort the Available libraries list by Managed Folders and select all libraries that come from "Eagle Pcb" in the list (Shift + left click). Now click the Use button. All "bundled libraries" that would come with EAGLE are usable now.
The same works with all libraries that are in the Fusion Electronics Managed Folder. Theses are the libraries that come with Fusion Electronics.
Thank you for bringing this up (again).
Best regards,
Richard Hammerl