@anidutta - Why do you want to install the rain sensor? The Rachio unit will use the weather station you've selected to determine if it should water if you have the weather intelligence and rain skip turned on. Rachio will look at the actual past 24 hour rainfall and future 24 hour forecast rainfall to determine if the schedule should still run. You can also turn on freeze skip to skip any watering when it is freezing and the runoff would turn to ice.
The MiniClik has three wires according to the documentation that I can see from Hunter, so I'm not sure why you're seeing just one wire.
It could be the MiniClik is installed in-line on the common wire and the remaining wire you're seeing is the normally open wire that is normally not connected to anything.
If you still want to install it, the link provided by @emil has the directions for multiple versions and as @robertokc said there is nothing unique about installing a rain sensor. And post a picture of the rain sensor so that we can understand the one wire.
Finally, the technical support folks at Rachio are top notch - set up a call with them. But, I really think you can do without the sensor.