All of the discounts are listed on the members-only website. If you didn't find your sign-in information in your paperwork, call Member Services and they should be able to give it to you over the phone.
Annual passes cost the same amount whether you buy vouchers in advance or just purchase the passes when you arrive.
The dining plan is available to members. Whether you get it or not is up to you. I believe it's $39 per adult and $11 per child, per night. You MUST buy it for every night of your stay. Many people say that it's way too much food over the course of an extended trip.
Another dining option is the Disney Dining Experience card. It is available to annual passholders and Florida residents. The DDE card has an annual fee ($60, I believe) but you then save 20% off of all purchases (alcohol included) at most Disney restaurants.