From your plots, it looks like the 3D plane containing your data could be of the form
a*X + b*Y + c*Z = d
but with c=0. Obviously, when c=0, this cannot be rewritten as a function Z(X,Y), as you are trying to do.
The following is a way to fit the plane without CFTOOL and it works for all plane orientations,
m=mean(data,1);
[~,~,v]=svd( bsxfun(@minus, data, m) ,0);
abc=v(:,end); %plane normal vector
d=dot(abc,m); %distance from origin
a=abc(1); b=abc(2); c=abc(3);
