Library management requires a simple but strict discipline.
Merely downloading a folder of objects to your hard drive does not automatically add it to the library database. [Imagine that Archicad doesn't have the time nor inclination to search all the crannies of your hard drive every time it starts up. It is not a hungry ferret.]
The Library Manager [File>Libraries and Objects]is the tool to add new folders to the library.
Once a new folder is in the library, its contents will become available to use in the project.
Further, it is best to have two categories of library: the built-in and basic libraries that come with each version, and a separate folder for everything else. Make sure both groups are added to the library.
Dwight Atkinson