Make sure that you have installed the AI/PDF Importer for your Strata software.
When you open the Design 3D CX 8 install disk image, you will see two drag-and-drop entries in the window. The first (top) one is the application, and the second one is the AI/PDF plug-in.
To use the drag and drop installer, just click on the icon on the left side, and drag it to the alias icon on the right side. Make sure you install the Strata Design 3D CX 8 application first, then the AI/PDF plug-in.
If you don't install the importer the first time, you can always go back and do it again. Just open the Design 3D install disk image, and click and drag the AI/PDF Importer to the Extensions icon on its right.
NOTE: For the AI/PDF plug-in install to work as designed, Design 3D CX 8 must be installed into your system (not user) Applications folder.