What you mean "it's not doing anything"?
I did a quick export and is working. This is a exported terrain from "Empty Terrain" or the sand level.http://i.imgur.com/Ehw99kI.jpg
I mean, it looks like it's processing something for a moment, but then no files are written.
What's also weird, is when it opens the file dialog to let me choose where to save it, what to name it etc.. it's an "Open" dialog, instead of a "Save" one.
Also, the image you posted looks more like a lightmap than a heightmap.. or is that one of the images exported with it?
Nothing is saved to the destination folder after I click the Export button