Unfortunately Excel 2013 removed support for that:
https://olappivottableextend.codeplex.com/wikipage?title=Excel%202013
You will have to put DAX calculated measures in your Power Pivot model to accomplish your goal.
https://olappivottableextend.codeplex.com/wikipage?title=Excel%202013
You will have to put DAX calculated measures in your Power Pivot model to accomplish your goal.