If your FEM-Design 3D Structure model file opens with the older version or it does not open at all when you double click on it so that because in the settings it is chosen to open with the old version or with (Unknown application) instead of the 3D Structure latest version. To solve this problem follow the steps below:
Solution 1:
Right click on the model file and go to Properties
Choose the latest version!
If it does not changes to FEM-Design latest version (fd3dstruct.exe) then you must try the next solution below:
Solution 2:
After repairing the FEM-Design if it does not changes automatically to the latest version when you double click on the model file then try Solution 1 again and it will successfully changes.