The "content center library files" is the storage location for parts that you use from the Inventor library when doing assemblies. They live in database form (like iparts) until you actually create one and drop it into your assembly. To make the actual parts you go into an assembly and where there is "Model" with a drop down arrow in the browser click and the menu should drop down with "Library". Select "Library" and that will bring up the contents library. Just use the eyedropper to insert into your assembly.
