You can change the material blend mode to Add or Screen. This works well for when you want to create wireframes, nebulas, or translucent objects that blend together (like seen in the Star Trek behind the scenes).
Adjust transparency of polygons the material is mapped to. (Note: Transparency is a bit of a limited function in Element due to some current gpu limitations and may not yield results like you would find in a full 3d program). Also see Render Order
for alternate looks when using multiple transparent objects in a scene.
Adjust the amount of smoothing for polygons associated with this material.
Visible to Camera turns off the object from being rendered in the scene, but cast shadows. Cast Shadow will turn off the object from casting onto other objects. Receive Shadow will turn off shadows cast onto itself.
Converts semi-transparent areas of the map into completely transparent or completely opaque areas. This works with PNG texture files that have an Alpha Channel. See Opacity Maps
for more info. Using the Add and Screen Blend Modes help when using Alpha Threshold as well.
Force object to render invisible but receive ambient occlusion or occlude objects that intersect. This is for when you want a ground Plane to catch a shadow so that it can render on your background image.
ADJUST POSITION FIRST
You may want to adjust the position of your floor object before turning Matte Shadow on since it will render invisible. Make sure you have Ambient occlusion turned on in the Render Settings as well to catch on the floor.
3D Truck Composite
Use 3D Camera tracking in AE CS6 and Camera Tracker
Setup and align ground plane and adjust 3D coordinates
Build Matte Shadow object (ambient occlusion)
Turns off Ambient Occlusion for model.
Increase Ambient Occlusion amount for model.
Increases Glow Amount for model. Glow is turned on in render settings.
See 3D Model Trouble Shooting for info on Draw Backfaces.
Two Sided Lighting
Allows the model to be lit on both sides of the geometry.