Well, since they've been no other responses...
I took a look at the website,
http://www.marquisspas.com/spa_silver_euphoria.aspand I don't see anything about the stereo being included. If you click on the Soundsations logo on the right, it says 'available on all 2005...'
1. Perhaps he said (and wasn't clear) it's 'on all 2005 models' meaning available now on all 2005s....
2. And, if you didn't get what you heard as 'included for $0' written on the contract as being a free option, well, what can we say, but, oops!
As for adding it, I'd think it would be custom work for the transducers, and I have no idea if they could mount the stereo control head (is it outside, or all by remote or what?).
If you have a dealer with a 'satisfaction guarantee', perhaps you can say that you thought it was included, the salesperson said so, and you want it so much that you'll pay for it and want to swap out the spa.
Good luck!