If you are going for a still image I suggest placing a bitmap on a plane inside the cube like Adam suggested.
You can make nice smoke with a light and noise too. There is a thread at c4d cafe that has a few example project files posted search Mograph Smoke.
Pyro cluster is found in the the materials manager. (default bottom of the screen)
materials: file: shader there you will find a volume tracer and pyrocluster… for pryro to show up you need to put the volume tracer on an enviroment object. Enviroments can be found in the lights drop down menu. Then place the pyrocluster on an emitter…play a few frames and render.
Inside the pyrocluster material you can choose smoke, fire and a few others.
/randy