To import 3D model in PADS layout:
Place the footprint in the PCB.
Open the 3D viewer by going to View > PADS 3D
3. Right click on the footprint and select Edit Decal.
4. Because the 3D viewer is open, a window will pop up titled Align 3D Models Under Parts mapped to selected model select the part you just added to the board
5. Click Import and navigate to the STEP file.
6. Adjust alignments as necessary.
This is what it should look like when adjusted correctly aligning the pins with the holes for example.
7. Go to File > Exit Decal Editor to go back to the board view.