Finding the data you want in the pivot chart (see red circle) is one good way of figuring out what function value you want to use. For many functions, the Object Name, Data Source, and Data Item in the pivot table define the expression to use in Simio.
In this case it would be: Server1.Processing.NumberEntered.
Of course you could also just use the expression builder and read the help on each pull-down item until you find what you are looking for.
VP Operations, Simio LLC