Can't evaluate/find by XPath

Is there some amount of setup required before I can use the Evaluate XPath tool? I have an XML file open, but the Evalute XPath button is grayed out in my toolbar. All searches I run using the Find by XPath tool fail as well, though I'm assuming/hoping that will be fixed when the Evaluate option works.

I can run the same XPath queries in Notepad++'s XML Tools just fine, but PhpStorm just says "No XPath Result found in Project."

Any ideas? Thanks