If you are going to build A using nested for loops, then you need to capture the result of each loop. You are currently overwritting A each time. See this page for more and examples.
Of course, with MATLAB, you don't need the for loops. The main thing you need to create a surface is for A to be a matrix. Take advantage of MATLAB's array capabilities.
B=-10:10;
L=-10:10;
% Transpose B to take advantage of implicit expansion to create the matrix.
A=B'+L.^2;
surf(A)
