VREF is basically the speed that you plan to touch down at, computed by your weight and flap settings.

To select a VREF, for both the 737 and 747, go to the Approach REF page on the CDU. To get there press the INIT REF button when decending or go into index and select Approach.

Now that you're in the approach page, on the right of the display there should be FLAPS15, FLAPS30 and FLAPS40 (or FLAPS 25 and FLAPS30) with there respective landing speeds. Click on the LSK (Button) to the right of the flap and speed setting that you want to use, now it should be written in the scratch pad (bottom of display). Now click on the LSK to the right of 'FLAPS/SPEED'. FLAPS and VREF, should be now be entered into the computer. Approach speed is typically VREF+5 or maybe more if it's gusty.

For a good landing, approach at VREF+5 until 30 feet Radar altitude for the 737, or around 50 feet Radar altitude for the 747, now smoothly idle the throttles. (Radar alt is called out by the plane). If Autothrottle is in use, it will retard throttles automatically.

Also. try consulting page 48 of '08 PMDG 737 Flight Management Computer', which should be found in your start menu or '12 FMC Users Guide' page 53, for the 747.