I don't think that they are omitting it. They are assuming that you understand that W = mg.
Wikipedia has the same formula for rolling resistance w/ mg notation, but they do not have a velocity in there.
http://en.wikipedia.org/wiki/Rolling_resistance
The weight issue is further compounded in the english system of units, which has two pounds (pound mass and pound force). This is explained somewhat by the guys at mathematica.
http://scienceworld.wolfram.com/physics/Pound.html