Download the effect here: http://specialeffects.imoviehowto.com/green-screen-effect-download-repository/

If it appears in the video that the effect is not there, it is there but the effect is very thin and transparent. When imported into a movie project and keyed out with a darker background behind it, you then will be able to see the effect better.

iMovie Rain Effect Tutorial: http://www.youtube.com/watch?v=7QgpU_h6Lhs