Autodesk 123D Design for Mac is no longer officially supported or available for direct download from Autodesk, as the software was retired in 2017 to make way for more advanced tools like Fusion 360 Status Report: Autodesk 123D Design for Mac Availability Status Retired/Discontinued
Since running 123D Design on a modern Mac is technically infeasible for most users, migration to a current alternative is necessary. Autodesk 123d Design Download Mac