Once the controller start, you can click on the first pushbutton (B1) and the LCD display will show the ip address assigned to it. You can also check your router’s configuration page, where there is usually a page that shows the list of all clients and ip addresses.
Your post title is ‘mac address’. That is different from ip address. OpenSprinkler’s default mac address is:
where the last byte can be customized by using the ‘Device ID’ option.