How to make a water fall have a silky look in picture?

I'm using a nikon SLR camera… How do i do it! I can't figure it out!

Keep the shutter open. Also you must hold the camera very still and make sure there's not too much brightness for it will obviously be intensified.

On a very bright day, a slow shutter speed to create a deliberate motion blur effect. You just need to get a Neutral Density Filter (and Filter) so the photo would not be overexposed. Thats the way you would get the Silky Effect you were mentioning.

Use a slow shutter speed

Slow shutter speed and higher fStop to compensate. This will have the added advantage of greater depth of field, helping to keep everything from foreground to background in focus.

Slow shutter speed and using a tripod is key.

Examples of some of mine:

http://www.altitudestock.com/image/I0000VdR1BuVonQ0
http://www.altitudestock.com/image/I0000jTmbox8KZR8
http://www.altitudestock.com/image/I0000zNtV6DhrQzw
http://www.altitudestock.com/image/I0000kIR15V0iGb8
http://www.altitudestock.com/image/I0000UQdLCjF1yhE
http://www.altitudestock.com/image/I000007_vQSHHwEg
http://www.altitudestock.com/image/I0000oTGmC_VclB0
http://www.altitudestock.com/image/I00005VPZQxXpsYE

A long exposure, anything from a few seconds up.

Dig into your camera bag and pull out your light meter. Take a reflected light reading off the subject area, in this case, a waterfall.
Or take an incidence light meter reading, (that is push the white dome or white flat thingy over the lens that takes the light reading of your handheld light meter), by turning directly away from the water fall. This kind of light reading measures light falling on to your subject.

Alrighty. Now I bet you are thinking of a combination to stop action. Which unfortunately for this application, (what you want to do) is wrong.
What you have to do is look at the other end of the aperture and shutter speed scale of your handheld light meter. It should be something like long fractions of a second… Like 1/8 second. And the aperture should be a big number which means you have to close the lens down.

You introduced a complication here. Not knowing if you are using a film camera or a digital camera you have to do this in this situation with either camera.
Use the slowest asa film or iso setting to obtain shutter speed readings of 1/8 second or longer with corresponding apertures you can use on your lens.
If you can't get your handheld light meter or cameras built in light meter to show you a decent shutter speed range, you have to do what the photographers of years past did and of today's photographers do. And that is to wait for the correct LOWER LIGHT lighting conditions This could be a very overcast day or nearer to sunrise or sunset.

Your cameras built in light meter I totally ignored for this teaching exercise. You need to use a slow speed color or black and white film or very low iso settings on your digital camera. You will be taking pics with very slow shutter speed settings of low fractions of a second or multiple seconds.

And you need a tripod and a shutter release cable or built in camera self timer to make sure camera is rock steady during exposure of your "silky looking water fall pic."

Here is the logic. Regular shutter speeds freeze action.
Long shutter speeds of say ( kind of short to longer exposure times) 1/8 of a second, 1/4 1/2, 1 second, 2 seconds, 4 seconds will make the action "flow".
Possible one shutter speed/aperture combination or maybe more will give you what you are looking for.