If i were to do it, i would start out by making your function, an actual function in Maple, like this:

f:=(x,y) -> x*y*(x^2 - y^2)/(x^2 + y^2):
plot3d(f(x,y),x = -1..1,y = -1..1,axes=boxed, numpoints=3000);

The way John explained should work just fine though, but i find it more logical like this...
But make sure you have: "with(plots):" at the top of your worksheet!

And for you Emilie, this should work (Assuming you have loaded the "plots"-package):

f:=(x,y)->(x*y*(x-y)) / (x^2+y^4):
plot3d(f(x,y),x = -3..3,y = -3..3,axes=boxed, numpoints=3000); 

Hope I was able to help just a little :-)

Sincirely
Dan