To send the model back to Inventor, first you have to convert the new geometry to the Body Object type.
Convert the new parts to body objects:
  Select the U-shaped portion
               of the new stirrup part: That is, the box with extrusions, but not
               the pin.
            
  Create panel and click  
  (Geometry). Choose Body
               Objects from the drop-down list.

3ds Max Design converts the part to a solid Body Object.

  Select the new pin cylinder,
               and repeat step 3 to convert it to a Body Object as well.
            Export the modified assembly as a SAT file:
  Application menu, choose Export.
            

3ds Max Design reports that the objects were exported successfully.
