Are you a DVC member? If so, you call MS to add the DDP to your reservation. You pay for it at check in.
At this time, MS does not sell tickets. You may buy those at check in or in advance - anywhere that sells them.
If you are renting a reservation from a member, you must ask the member to call MS to add the DDP to your reservation. You pay for it at check in. You are on your own for tickets, but you may buy them at check in if you like. Most buy in advance to get a discount.
Member or not, you must add the Disney Dining Plan to your reservation at least 48 hours prior to arrival. You cannot add it at check in.
You specify that you want the DDP when you make your reservation with MS. Or you could add the DDP onto an existing reservation up to about 3 days before your vacation by calling MS. You pay for the DDP upon check in.
The only discount that DVC members currently get on park tickets is for annual or premium annual passports. The discount amounts to about $100. You can purchase them online at the DVC website or at the parks. It's the same price either way.