Add a velocity envelope- right click on one of the little "nubbies" at either end of the clip. A menu pops up, letting you adjust the speed. Minus (-) is reverse.
Where in the event are you setting the velocity envelope to -100%? If you are setting it at the beginning of the clip, there is no clip footage available to reverse. Split the event at the time where you want to clip to play in reverse and set the velocity envelope to -100% at that point.
In VV 4b right click the clip you want to reverse. From the pop up menu you select "Insert/Remove Velocity envelope". Right click on the handle at the start of the clip's newly added velocity envelope. Click on "Set to 100% Reverse Velocity."
You don't need to type in anything doing it this way. The preview my take a minute to kick in but it will sort of do a stop motion looking preview. The audio will still be played forward though.