Online check in opens 24 hours before departure. We fly on the 2nd Aug and can check in from 10:25 on Sunday morning.
After putting something like this on as well, I was advised to try from between 8-9am.
Yes you do do it through manage my bookings, go in put your res number in and surname and it should come up with your details, here you can book special diet meals, kids meals, seats etc.