That’s so very awkward…
You can’t get PowerPivot 2013 with Office Professional 2013. It’s available only with Office 2013 Professional *Plus*. Oh wait a moment. You can’t get Office 2013 Professional Plus just like that. It can be get ONLY with Office 365 subscription.
I do not know what to say about this. Just plain illogical for any customer to go for an Office 365 subscription JUST for PowerPivot and get screwed with monthly/annual subscription fees. Mark was so very right. PLUS stands for Please Let Us Screw you.