Here are your parts includes a wiring diagram.
Replacement parts for DU4500B models | AppliancePartsPros.com

Where do I find that valve and how do I turn it back on?
The valve is Item 2 in Section 6. It is behind the kick plate under the unit.
There are a number of things that can stop water flow.
1. You will see a switch under the unit left hand side, this is the overflow switch. There is a float inside the tub, if the unit overfills the float rises and deactivates the switch. This disconnects power from the fill valve. Sometimes due to crud etc. the float can stick in the up position so you have no fill.
2. On the input side of the water valve there is a screen filter. It can become clogged with crud and stop the water flow. Remove the valve and check/clean the screen. Do not remove it as it is there to prevent crud from getting into the valve, holding it open and causing a flood. When replacing, shut off the water supply, put a tray etc. under the valve, then slowly loosen the connection till it starts to drain. When the tray is full tighten the connector till no more dripping and empty the tray. You will probably have to do this a number of times but it will cut down on the amount of water on the floor.
3. The valve is shot
4. the timer is shot and not sending power to the valve.

Easiest way to troubleshoot is with a meter.
Measure across the valves contacts when it should be filling.
If you see 120 volts either the valve is shot or the filter is clogged.
If no voltage
With power removed from the unit, check the overfill switch for resistance, should be 0 ohms. If not then either the float is hung up or the switch is shot.
If all OK then probably the timer is shot or one of the wires from the timer to the valve is broken/open. You can also check this with a meter by measuring from the power Neutral side of the valve (white) to the timer side of the overflow switch, should be 120 volts.

Here is a good site with troubleshooting help
http://www.applianceaid.com/dishwash.html