I know about this one from a few different ways on both sides. All big supermarkets and even stores like Wal-Mart have people called Category Managers and sometimes people called buyers and merchandisers. These may be the same person or they may work as a team. These are pretty high pressure jobs and these people manage a given product line in the stores. The have to run it almost like their own business in many ways.
Lets say you have a new line of butter that you want a medium sized supermarket chain to sell. The most direct thing you have to do is to get a meeting with the dairy category manager. This isn’t all that easy because there are lots of people trying to sell them things all the time. You have to “sell” your product to them with all the great reasons why they should carry it. They will hardly ever take something on the first try. It may take months.
Once they express some interest in carrying it, you have to work out details and these people are good at extracting money wherever they can. As mentioned in the other thread, it is likely you may have to pay money to get the chance at all and they reserve the right to discontinue it. Even after they carry it, money still flows back and forth in ways that make Enron look simple.
After they accept your product, you still lots of responsibilities as a vendor. You will have to pay them to put your product on special for example.
It is a brutal business and they have the upper hand. One big myth floating around is the supermarket margins are razor thin. That is generally false and they have money flowing in from lots of directions.