For the 767. Use the RDR from the SimIcons Panel (Shift+2) or use the RDR panel in the VC on the Pedestal to turn it on. You will see it in the HSI Display.

The radar is already there in the 727 if you are using version 2.3. To turn it on, right click on the button on the bottom left (in the place where it looks like there is room for it) three times and wait for it to start.

You need to adjust the GAIN and TILT to see the weather in both of them.

Mark