As Gerry states in his comment, the graph(s) containing the prerequisite custom legacy library nodes need to be imported before any graphs that utilize those library nodes.
The steps required are included in the 'Importing legacy documents' section of the Data3Sixty Analyze Migration Guide that can be obtained from the Downloads section of the forum (either from the Desktop edition or Server edition download pages - sign in required).
The Help also includes topics on importing legacy documents (see Getting started > Importing and exporting documents > Importing legacy documents in your local Help or the corresponding online help topic.) Information on library paths is also available in the Help (see Advanced topics > Library paths or the online help topic for the latest version of the product).