how to a real time control between ardunio and simulink model based on gui ?

function p_button_Callback(hObject, eventdata, handles)
% hObject    handle to p_button (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
a=get(handles.edit_pGain,'String');
load_system('water_level');
find_system('Name','water_level');
open_system('water_level');
set_param('water_level/PID/Pr','K',a);

we can change the value of the simulink model by using this gui code but it's nor working in real time. We wanna the axes1 will show the changing in real time that includes the parameters changing in real time. Please help me about that. How to write gui code? Thanks