How to book an excursion on a theme cruise

I'll be on the Celtic Thunder theme cruise Nov.8/14. The Celtic Thunder booking team advised I can book my excursion in Nassau direct through the MSC cruise line. So I went to their website and put in my booking number and it tells me that I have booked through a travel agent and to contact them. Any one else going on this cruise had any success with booking an excursion and, if so, how did you do it?

Thanks!

Your cruise is too far away to book excursions. Check in September.

I think the Celtic Thunder theme cruise is a private full charter which means when the promoter hands the final payment and manifest over to the cruiseline (depending on the contract 14-30 days before sailing) you will be emailed by the promoter a cruiseline booking number (meaning you are now in the cruiseline's system) at which time you can book your excursions, spa, etc