It looks like to open .mlx in 2013a is not feasible. Probably you already know that if you can save it using newer matlab version in .m file and then execute it in matlab 2013a.
You can read this thread given by MATHWORKS staff: https://www.mathworks.com/matlabcentral/answers/271486-are-live-scripts-compatible-with-previous-and-later-versions-of-matlab
