Pivot table: median instead of average

Dear all,

I have made a couple of pivot tables with price data. Unfortunately in the value tab I can only do average of price and not median of price (see attachment).

I have read that the power-pivot add-in can do the trick, but for some reason in my office 365 version for mac there is no option button when I open excel to activate the plug-in. Also when I do Insert --> Add-in --> Get add-in, then go to the stores, I can't find power-pivot there.

Would be great if someone can help out...