I tried your file. It's pretty cool. Especially if you turn the particle rate up to about 200 or so!!
Here's what you can do so that you can see the rain drops when you render.
First, You're right; you have to apply the material to an object. The fluid effect is a texture that feeds into a blinn material (blinn4 in the hypershade window)
Make a plane and assign the blinn4 material to it. You can right click on the plane and choose materials>assign existing material>blinn4.
Now the plane will have the raindrop effect on its surface. I think you probably tried this and it didn't work for you. Right? Well, the effect will be visible in the work area after you enable 'hardware texturing' under the shading dropdown above your work area (same place you turn on X-ray and stuff like that).
When you render, you have to use Maya's hardware renderer.
Today is the first time I played with the fluidtexture2D texture so I don't know much about it except that it acts just like any other 2D fluid container. All the same settings... I don't know why but the Blinn4 material doesn't turn out exactly like the 2D fluid effect. It looks similiar but kind of like it's zoomed in and kind of blocky. I played with some of the setting under the 'place2Dtexture' node but I couldn't get it to look any better. Maybe you have to use the 2D texture info to create displacement.
Give it a try and see what you can come up with.
I think the file you sent is cool. It looks like the particles that are emitted collide with the plane right below them. There is an expression that seems to be using the collision info to disturb the 2D fluidtexture. I don't really understand how it's all connected but I plan on playing around with it more sometime later.
Another way to do the same effect is to instance small NURBS or Poly spheres to the particles and have them collide with a regular 2D or 3D fluid container. This way you can get the same ripple effect but also get the fluid to splash if you want. The file that I got from you is using info from the particles colliding with the plane below them because particles don't collide with fluids BUT NURBS and Polys do!
Fluids are a pain in the ass but they start to make sense after a while. They can do a lot more than just make smoke and fire