Attached is a project that shows how to start and stop the sprite emitter.
I can't really explain much.
In order to animate the material(Sprite emitter) you must click on the "Show more than drivers button"
It is located right next to the "Shortcut to <Model name here>"
in the project workspace under the choreography
the project file should work I embedded all files.
Hope this helps
Uploaded the corrected project file
Here:
example.mov
Missle.prj