What is Candy's inclusion in George and Lennie's plan?
Candy shares the general plight of the iterant farm workers in Of Mice and Men - all of his years of labor and sweat have not earned him anything of his own. The prospect of having a farm of his own, working alongside his friends, is exactly opposite to his current state as we find him in the novel.
Looking at Candy’s practical role in the shared dream of George and Lennie, Candy says that he will cook and clean and tend the garden, essentially reprising the role he already fills at the ranch. He will also pay a large portion of the down payment to buy the land and the house.
If we look at what the dream means to Candy, the answer is less practically oriented. While George and Lennie have each other to talk to and to rely on (and to look after) Candy has no one. His dog was his only friend, companion, and, by extension, the only thing in the world that Candy could call his own. When his dog is killed, Candy is left with nothing to show for his life - no friends, no possessions, and no future.
The dream provides an answer to these particular poverties for Candy a sit does for George and Lennie. Candy, however, has even less in the world that belongs to him than George and Lennie do. The dream of working on a ranch of his own is also a dream of finding lasting friendship.