When Diamond talks about proximate and ultimate causes, he is trying to explain what has caused Europeans to become dominant in the world. Proximate causes are those that led immediately to this dominance. Proximate causes include the technology (guns and steel) that allowed the Europeans to defeat people like the Incas.
But Diamond wants to know why the Europeans were able to develop the technology. This is where the ultimate causes come in to play. These are factors that allowed the proximate causes to come into being. The ultimate causes that Diamond is talking about are things like the length of the east-west axis in a given place or the number of species of plants and animals that could be domesticated in that particular area.
Proximate causes, then, are the things the led immediately to European domination. Ultimate causes are those factors that caused the proximate causes to come into existence.