Edit the pivot table and go to Expressions tab. There, at the section Total Mode there are three options: "No Total", "Expression Total" and another with default "Sum of Rows". You have to select the third one and change "Sum" for the operation you want.
If no one of the options shown fits your requirements, you can always create a new expression for calculating the mean.
I explain my problem, I have some customers with a lot of value and I want the mean for each of these . . . The pivot table give me the total but non the mean, instead of the Straight Table give the mean of all customers . . .