Ok here goes.
You plumb your RODI unit to your water line (not through the actual faucet) use on of these.
http://buckeyefieldsupply.com/showproducts.asp?Category=187&Sub=133
After that connection you use a solenoid valve. A solenoid is a valve that opens and closes electronically. The one used for this application remains closed at all times unless it gets power. Once it gets energized it opens.
http://buckeyefieldsupply.com/showproducts.asp?Sub=508&showspecials=508
So you have your water line which is plumbed directly to your RODI with a solenoid valve on the line between the water line and RODI unit. You plug in the solenoid valve, it gets energized and water flows to the RODI unit.
Now you get a bin for top off water and install one of these.
http://buckeyefieldsupply.com/showproducts.asp?Sub=518&showspecials=518
You plug the high low switch into an electrical outlet and the solenoid into the high low switch. The high low switch works as follows. You set the low switch somewhere near the bottom of your bin and the high switch somewhere near the top where you want the water to stop. When the water level gets to the bottom switch it will energize the solenoid valve and water will start filling the bin. Once it gets to the high switch it will turn off.
It's always a good idea to use a mechanical float valve as well in your top off bin as a safety precaution because float switches and solenoids can fail from time to time. FYI you don't have to use the high low switch that Buckeye sells if you have an apex controller you can get a break out box and 2 float switches and program it to do the same thing. It's also a good idea to put an auto shut off valve on your RODI unit. In case the float switch fails the mechanical valve will stop water from entering the top off bin and prevent a flood but the waste water will keep running and that's a waste of waste water

. You can also get one of these to have the waster water go directly to your waste line.
http://aquarium.bulkreefsupply.com/search?asug=&view=grid&cnt=16&w=drain+saddle
Since it automatically fills up every time it gets low you don't really need a large storage bin anymore. A 10 gallon tank will be more then enough. It will just get filled up more frequently. That being said making larger quantities of water at is usually better for the membrane and produces better water, but space is something to always consider.
I HTH
Adam