PaintFX are already part of Maya You can use them to paint on a canvas (2d), your viewport (3d), or a Nurbs object (also 3d) . If you want to canvas paint, all you need do to call up the screen is to hit the "8" key on your keyboard. Then you will see relevant menus. Down below in the tut section I wrote something up on canvas painting you may want to look at.

For using PaintFX on a Nurbs you need to first make the object Paintable. You will find that option under the Paint FX menu, it's under the Rendering pulldown. Then you go to visor which is where all the brushes are located, you also want the Edit Template Settings window so you can change settings as you go. With PaintFX you can choose to paint an object or the viewport, for rain I would paint the viewport, so make sure that is checked.

That was kind of quick and not very thorough, the manuals and online help have far greater info

Darkon