dwname needs to be a cell, not a character array. Put it inside braces like this {dwname}. Or better yet do it this way where I first create the cell array and then write it out just once at the end instead of in every iteration:
% Create a 1 by n cell array called dwname.
n = 5;
for i = 1:n
dwname{i} = {sprintf('dw%d=-dudx%d-dvdy%d;',[i,i,i])}
end
% Write out the cell array all in one shot.
sheetNumber = 1;
range = 'a1';
xlswrite('name.xls', dwname, sheetNumber, range);
