You could download polyfitn from the file exchange.
But if you already know how to use the curve fitting toolbox, then just specify a custom equation of the desired form. Something like this:
modelform = fittype('p00 + p10*x + p01*y + p20*x^2 + p11*x*y + p02*y^2 + p30*x^3 + p21*x^2*y',...
'coeff',{'p00','p10','p01','p20','p11','p02','p30','p21'},...
'indep',{'x','y'},'depend','z');
It will be treated as a nonlinear model internally as a function of the parameters, but since all of the coefficients actually do enter linearly, they will be easy to fit. fit should have no problems.
