The code in the GUI Callback creates variables in the callback function workspace. Do you run the simulation from the function or independently? If you run it using the SIM command from the function, use this syntax:
options = simset('SrcWorkspace','current','DstWorkspace','current');
sim('mymdl', [0 duration], options)
Otherwise, make sure that the variable 'wArray' is created in the base workspace, because that's where the model looks for it:
wNewVal = str2double(get(hObject,'String'));
w=wNewVal;
wArray=[w w 0 0 -1*w -1*w 0 0];
assignin('base', 'wArray', wArray);
set_param('SDH/Tp/w','Table','wArray')
