Merrill, huh? Here we go...Pick up your bass. Plug it in. Turn on the amp. Now, remove your left hand from the bass, you won't be needing it. Go ahead and hit the E string a few times. There you go.
JK, of course, just had to have some fun with the generalizations. I actually rather like Robbie's sound (Godsmack is one of my favorite bands right now) and a lot of his basslines do just what they need to...and Voodoo is just a sweet little groove.
AFAIK, his sound comes primarily from his Ibanez basses (and a couple spectors, I almost always see him playing ibby's though) in drop D through a wall of amps/cabs.
Here's something to help you along your way some, sorry I don't know too much more off the top of my head, I haven't researched him too much:
http://guitargeek.com/rigview/378/