[Jared Russell]

The grayhill encoders I use (and I believe the US Robotics ones as well) use 5V power.

As for mounting, it depends which type you got. If there's a small shaft on it, you can either couple it to the end of a driven shaft (in the gearbox or on the wheel, assuming the wheel shaft isn't free spinning), or put a small gear/sprocket on it and run it off of your existing geartrain configuration (this is what Miss Daisy does). Then, use the threaded shaft cover to tighten a nut between the encoder body and the shaft with a thin piece of metal in between.

If it's a hollow shaft encoder, just slip it over the end of a gearbox/wheel shaft and secure the body to the frame using the mounting holes.

Remember, if you mount an encoder somewhere other than on the actual output shaft, you have to take into account the gear ratio to convert to standard units (which is optional).