All we did was ask a CM at the Main Street Station - if you do a search, there are quite a few threads on the subject, as I know a lot of people have gotten the chance to ride by taking tours and other things.
i went there on my 30th birthday, and we were on the train and i just asked one of the CM's and she was soooo nice, she radioed in to see if i could and she let me.. it was an amazing ride.. i took alot of pics too!!
Oh well i recently got on and all I had to do was ask nicely, It might take a few try's(sp) but they might tell you to come back later and even then they can't garrantee(sp) a seat. If you're a club 33 member I think you can go on whenever you want to but basicly all you have to do is ask-and you also can only get on the lillybelle at Mainstreet station(it's a rule). And if a club 33 member is on it than they have to be ok with it in order for you to get on. hope that helped