the glycerin would help to make it even more pliable and perhaps give it a bit more sheen. it should be available at a pharmacy if you want to use it. i don't think it's necessary, and in fact haven't heard of using it for fondant.
however, there are "fondant" recipes which are not really fondants. most likely, your recipe is one of these. essentially confectioner's sugar with corn syrup and maybe shortening. it's called a "rolled fondant" or sugar paste.
if you're a novice baker trying to do a nice cake, i might advise sticking to your recipe. however, if you're planning on using the fondant to stuff something, you might want to go ahead and use rob's recipe above. with a little experience with a true fondant under your belt on a more forgiving product, you might be ready to try it on a cake next time around.