On the ArchiCAD Help Menu, beneath the first two items, you will find two lists of items.
The first list consists of links to PDFs, including the ArchiCAD manuals. The PDFs reside in the Documentation folder within the ArchiCAD folder. Any PDF placed in this folder will appear in the menu.
The second consists of links to web sites. The links reside in the WWWLinks folder within the ArchiCAD folder. Any link placed in this folder will appear in the menu.