Dear Runsheng,
If Simulink looks for the variable z and cannot find it, you could initialize this variable either from a constant block, the workspace, an m-file or direct in the initialization step of the model.
If you like to initialize z from the workspace, use the 'from workspace' block. Set the block parameter variable to z, and use i.e. the matlab prompt to define z.
If you like to initialize z at the start of the program, go to file->model properties->model properties->callbacks->InitFcn. Here you can type in z=1 (in Example)
If you like to initialize z at the start of the program, go to file->model properties->model properties->callbacks->InitFcn. Here you can type in initialise. If you now make an m-file which is called initialise.m where the value z is stored, you can initialise the simulink program over a mfile.
Good Luck, Christiaan
