plotting f(x,y)=A*(x/(y+x)) ?

Using createmesh as Fred has shown sure is a good way for doing the job. Especially because you can see (and therefore automatically document) and easily can change the plot limits and grid number in you document.

Another way would be to simply use the function name (in Freds file thats f) WITHOUT arguments and parantheses as the argument for your 3D-plot. But you have to take care now that you limit your plot range in the format menu of the plot object under "QuickPlot Data", so you do not run into a divison by zero error. Unfortunately plot objects do not ignore that kind of errors. So to duplicate Freds example you would enter under Range1 0.001 for start, 5 for end and 20 (the default value) for grid# und for Range2 0, 5 and 20 respectively.