@Anwar Alm - you could use str2func to convert the function string to a function handle and then evaluate in your code. In this case, the callback would be changd to
function func_compute(~,~,hEditField,hAxes)
y = str2func(['@(x)' get(hEditField,'String')]);
n = str2double(y);
plot(x,y(x));
end
Note how the @(x) is prepended to the string that we obtain from the edit field (see anonymous functions for details).