you would first create a pushbutton in either GUIDE or manually using uicontrol. Then you would change the callback function to run a function that uses uigetfile() and the run() functions. see my example:
function test() %main function
%create the button and adjust the callback
btn = uicontrol('Style', 'pushbutton', 'String', 'Clear',...
'Position', [20 20 50 20],...
'Callback', @runfile);
%what the button does when clicked
function runfile(hobject,event)
[file folder]=uigetfile('*.m'); %prompt user to select mfile
run(fullfile(folder,file)); %fun the m-file.
