Open your .j3o and in the editor, you can manually select the tower geo and apply translation, rotation and scale to the model. This way, when you load the model to a node, the scale, rotation and translation is already defined.

To get the tower model, you must first cast the tank to a Node. Assuming the name of your tower model is Tower:

((Node)tank).getChild("Tower");

Lastly, rotation is in radians, not degrees. If you want to rotate using degrees, do: