I am very new to Painter and own Painter X, but have found that painting fur is much easier using a brush where you can see the bristles in the strokes but doesn't smear other strokes into a solid blob of color (as with Painter X, I use the RealBristle Brushes).
But as to the technique of making realistic fur. . .I suppose that answer would have to be. . .layers. Keep building on the layers of colors, and vary them a bit, even by slight hues. With your wolf painting, the colors in the fur seem to be pretty much the same with the adult wolf (the pup has nice shading though).
Wolves have thick fur, and looking at some Googled pictures of wolves, so I suppose you could mimic their coats with short, dense brushstrokes.
Cher Threinen-Pendarvis has a lovely Painter painting of a collie dog that vividly illustrates a dog's (in your case, wolf's) thick coat that maybe you could look at as an example of realistic fur.
Have a varied touch to the pen while you paint and follow the direction of the wolf's fur. Studying the subject helps a lot.
Looking through and fiddling with the airbrushes (I'm still playing around with Painter), what variant(s) of airbrushes do you use? The Soft Airbrush variant (this is in Painter X) looks like it could be useful in starting to build the fur. Maybe a Soft Airbrush and then some sort of bristle-ly brush to make the fur fuller? I found some airbrushes that make a bristle-ly look, but they end up in a solid color once you start layering. . .
Not sure how much this will help, but I'm sure someone more experienced then I could offer some more advice.
Cheers and happy painting!
EDIT, 8/13/08: The painting of the collie dog isn't by Cher Threinen-Pendarvis, but Don Stewart; it's found in Cher's Painter X Wow! book though. Just wanted to fix that.