For stereo audio, your data has to be an n x 2 matrix. Then you can multiply each channel (column) by a weight between 0 and 1 to pan.
% Full pan left
data = rand(100,2);
data(:,2) = 0;
% Full pan right
data = rand(100,2);
data(:,1) = 0;
% 25% left, 75% right pan
data = rand(100,2);
data(:,1) = 0.25 * data(:,1);
data(:,2) = 0.75 * data(:,2);
