Here, your code could be something like this. Keep in mind variable scope in functions. If the specified variables do not exist in your callback function, you'll need to pass them in or create them inside the function. Since I don't know how they are created, i've left that open ended here.
% this is my save button
function SaveGlobal_Callback(source,eventdata)
Mass_Global = ...;
Damp_Global = ...;
Stiffness_Global = ...;
GlobalMesh = ...;
[file,path] = uiputfile('*.mat');
save(fullfile(path,file),'Mass_Global', 'Damp_Global', 'Stiffness_Global','GlobalMesh')
end
