If your Smart Smoke Alarm is already paired to your account, you can find the MAC address on the settings menu. Choose your Home, then your Smart Smoke Alarm and the app will display the MAC address/serial number.
You can also find the serial number at the back of the Smart Smoke Alarm. To do so, remove the Smart Smoke Alarm from its mount by turning it counterclockwise. The serial number is at the right of the grey battery, after the text field 'SN=' and appears as 'gXXXXXX'. X being either a figure or a letter.