Go to your reservation: https://www.nwa.com/cgi-bin/res_info.pro Under the "Update Reservation/Add Information" header, select the drop-down option labeled "View/add special service requests." From here you can select your menu selections for each passenger in the reservation. Be sure to tick the correct boxes below for the applicable flights and click continue.

Once you have entered your information you can repeat the process to make sure your selections were accepted by the system.

Vegetarian is better, by the way. I am lactose intolerant, so I always choose the vegan, which is never as good as the vegetarian option.