You cannot book excursions before the 60 day mark. All excursions are booked thru
DCL (on line, fax, or phone number as above.) You really don't have to book at 60 days--it isn't like getting a table at the Cinderella castle. While it is advisable to book ahead if there is something you really want to do, it is frequently possible to book even after boarding the ship.
Also, be aware that many excursions can be done privately "on your own" at a much lower cost than DCL charges. Doing them on your own can also avoid the crowds. Of course, going thru DCL is majorly convenient.