The WXR is ON by default. Use the LEFT tilt button, on the Aft Aisle Stand (see the ‘777 Captain’ FLIGHT MANUAL Part II – Aircraft and Systems (csx777_manual2.pdf) page 129: P20. WEATHER RADAR PANEL P20), to adjust the angle up or down until you see the green/yellow/red of the weather (depending on the weather conditions).

Using "fair weather", you should only see green, but switch the weather to "building thunderstorms" and you should see all three of those colors. ;)

If you are using real weather (like AS2012/REX), then you need to find where there is bad weather to test the WXR. ;D