for that you have to right click on your transporter in your class library and click in "Edit 3D Properties". In the window that opens you have to go to the "MU Animation" tab. Here you can set under "Area settings" the orientation and position of the MU on your transporter.
I'm not certain what exactly you mean with assembling a part.
Is the part an MU that is transferred somehow onto your transporter? Or is it a graphic you want to become part of the transporter?
In any case, I think it might be best if you were able to post your model. Guessing this based on a picture is a bit challenging.