You have hundreds of old .123dx files from years ago. You need to edit them, but you cannot run 123D Design.
However, as Autodesk's portfolio grew complex, they consolidated these features into newer platforms: The web-based "introductory" successor. Fusion 360: The advanced, professional-grade alternative. 123D Apps: 123D Design, Sculpt+, Make, and Catch | Autodesk Autodesk 123d Design Download Mac