To import a 3D model in Proteus:
In the PCB Layout, select the footprint you wanted to add the 3D model.
Right-click on the footprint and click 3D Models...
3. A pop-up window will show. Select the 3D Mechanical Model tab and choose M-CAD (STEP or IGES) File.
4. Select the folder icon shown below the image and navigate to your downloaded .stp file for the part and click Open.
5. Once imported, you can now adjust the orientation of the 3D model until it fits in the footprint. Then click OK.
6. On the toolbar, select the 3D Visualizer icon.
7. Notice that you have successfully imported the 3D model.