Normally the problem is that the link to the external editor in Lightroom 5 points to the main PSE executable, when it should point to the Photoshop Elements Editor executable file.
Either that, or space aliens have abducted your photo.
the link in Lr5 to PSE 12 is the only one available - btw I'm using PSE 8.2 on both Lion & Mavericks - same results
You have to go into Preferences and then External editor and then set up PSE as an additional external editor, pointing to the PSE Editor application executable.
I don't want to seem flippant, but that's exactly what I did - Actually I set up 2 presets here (one for Adobe RGB & one for sRGB).
As I stated previously, I set these up on 2 different computers (a MacPro and a MacBook Pro), each computer running a different version of OS-X (Lion & Mavericks, respectively).
I have successfully set up several external editors using various stand-alone programs - they all still work -- with the exception of Photoshop Elements 12.