Find Your Cruise
This link should take you into the Celebrity Mangage My Booking, put in the Celeb cruise ref your name ship and the date and off you go you will see the excursions and be able to book-in online as well. Due to the length of the web address I had to put it into two lines but it should work for you.
The web address above is not allowing access but this one seems OK then when open select Manage My Reservation and below it Check-in online.
If you are talking about shore excursions - there are NONE to prebook on this cruise - all is included in the price, and you just sign up for the one you want the night before. I did this cruise last December so if you want any info just ask away...