To import a 3D model in KiCad:
In the PCB Editor, select the footprint you wanted to add the 3D model
Right-click on the footprint and select Properties...
3. A pop-up window will show. Select the 3D Settings tab.
4. Select the folder icon shown below the image and navigate to your downloaded .stp file for the part and click OK.
Note: For older versions, you may not see the folder icon. Simply click Add 3D Shape button.
5. Once imported, you can now adjust the orientation of the 3D model until it fits in the footprint. Then click OK.
6. Go to Viewer > 3D Viewer and notice that you have successfully imported the 3D model.