You don't need to book it through your
travel agent (although if they are a full service agency, they can certainly do it for you if you like).
DCL opens up bookings for excursions online based on your
Castaway Club status. You have to be paid in full to book excursions. If you are Concierge guests or Platinum CC, you may book 120 days before sailing, Gold CC is 105 days before sailing, Silver CC is 90 days before sailing and new cruisers may book 75 days before sailing.
Go online to your reservation (you'll need your reservation number and a birthdate of someone on the reservation). You'll select Activities and then port adventures. If the excursion you want is available, you would select it from the list, select the people participating in it and then book it. You then have to confirm it. You don't pay for your Port Adventures until you are onboard (they are billed to your onboard account) and tickets are left in your stateroom. You may make changes online up to 3 days before sailing. After that, excursions become non-refundable.
Hope that helps.