Okay, you have three different terms here, so let's make sure we define them before starting to answer the question:
- PLOT--the course of events that occur during the story,
- CONFLICT--what the protagonist wants and the antagonist wants to keep from happening,
- THEME--the general moral of the story...what the author is trying to say about something.
The question, then, is about how the story-line, including it's conflict, supports the theme in "The Monkey's Paw." Next, I guess we have to figure out each of the 3 in regards to the story.
The theme of the story (well, at least one of the major ones, there are quite a few you can dig out of this old chestnut) is "be careful what you wish for." This is supported by the fact that all the wishes made have an evil consequence to them (even the seemingly positive "last wish" sends the boy home, devastating the mother and solidifying the ruin of the family.) So how do the plot and conflict support this?
Well, the plot is strongly driven in this story by the conflict, so it makes sense to stick with that. The conflict of the story is the protagonist's desire to enrich himself through magical means vs. the monkey paw's desire to do evil. This simplifies the idea a bit, but I think that's okay in this case.
So how does that support the theme "be careful what you wish for?" Well, the monkey paw's mechanism for working its evil is through the fulfillment of the wishes. This is in direct opposition to the White's desire to use the paw to make their lives better. If the paw delivered the wishes as Mr. White wanted, well, there would have been no conflict and therefore the theme would not have been supported.
Indeed, the other plot elements support this theme as well. The idea that the boy would come back from the grave all messed up, or those horrible moments when the mother is trying to get the door open and just can't seem to work the lock, these plot elements help to support the theme as well.