I'm having the same N/A issue. I *did* have the trim www and index option checked, and even had mint set to domain.com, despite redirecting all incoming visiters to www.domain.com.
Google knows about www.domain.com, not domain.com. So it's not a surprise that when prank grabs URLs from the mint database without the www that they'd show up as having PR 0. But I've added a debug URL, my root level of www.domain.com, and it's still showing up in prank as N/A.