How do I read code for check engine light on 1995 Mercedes-Benz E320

When I push down and hold the button in the small box under the hood it does not blink, what should I try next to read code.

Asked by for the 1995 Mercedes-Benz E320
have it scanned with a prof scanner
Thank you for your response ,also when I'm holding down the button should the car be running or not.