You will also need a pump to move your rodi water into your sump.
I actually ended up buying the Tunze Osmolator 3155, but I did do the research on setting up my Reefkeeper elite to do exactly what you are trying to.
The theory is the float switch is on/off, if the sump is filled to the correct level the switch is on I believe. In your reefkeeper there is a scenario you setup that says if the switch is in ON position to cut power to the power plug you have your rodi pump on. If the switch is in the OFF position then the pump is to turn on.
Thats about it, as a safty though you would also want to set up that same scenario so that the pump will only run for x amount of minutes just in case your auto top off switch gets stuck.
I would also suggest using some sort of smaller type container for your RODI water that you manually fill you wouldn't want something to break and it to dump 55gallons of fresh water into your tank
That is one of the reasons even though I have a reefkeeper i decided to go with the tunze. It has two checks for water level. The idea i had was to eventually get the switch for the reefkeeper as well that would give me 3 safty checks to help prevent anything bad from happing.