Problem controlling arduino with simulink

I want to simulate arduino with simulinkand when i click run i have this error Error occurred while executing External Mode MEX-file 'ext_comm': Failed to connect to the target. A time-out occurred while waiting for the connection response from the target. Possible reasons for the time-out: a) The target is not switched on. b) The target is not connected to your host machine. c) The application for the model is not running on the target. You might have clicked the Stop button. If the Run button is not dimmed, click it. Otherwise, click the Build button, which downloads and runs your application on the target. Component:Simulink | Category:Block diagram error Unable to connect to the 'Arduino Uno' target for 'arduino_led'.