You can book online, but it's a slow and mostly one-way process...
Basically, you send MS an email that contains a form (on the member website) that lists your first & second choice dates, resorts & size accommodations (if you don't have second choice, you just fill in your first choice in the second choice boxes as well as in the first choice boxes).
Almost immediately, you get an email back from MS saying they'll get back to you in 2-3 days.
Then, in 2-3 days they either tell you you got your reservation (and its specifics), or that you don't (and if you don't, why you don't).
You can't check availablility online (that I know about anyway). And you have to be a member to access the members services site.
I've booked online 3 times...twice it went thru smooth as silk; the other time I got an email telling me they couldn't book my requested time and why (I was outside the 7-month window and asking for a non-home resort--it was my first attempt and I forgot about the 11-month window restriction...they were really nice about it).
I love online booking for "easy" reservations...far enough out at easy times when I have no questions...
But you need to call MS if you have availability issues or if you have time issues...
Good luck!