I'm running Windows 10 with Gimp 2.9.6 and Rawtherapee 5.2. I don't know what I'm doing wrong, but I can't get Gimp to open a RAW file with Rawtherapee. The release notes and ducumentation says it is now possibe but I'm stumped. I tried edit, preferences, image import but it doesn't show the Rawtherapee plug in nor does it allow me to change the raw place holder which just displays the error message. I've been able to configure Rawtherapee to launch Gimp but I'd rather start with Gimp and launch rawtherapee. Any suggestions?

I had to be told that one, apparently it is obvious

The last line here http://rawpedia.rawtherapee.com/GIMP_Plugin

which says

Usage: Just open a raw file from GIMP. A RawTherapee editor window should open automatically, which you can use to tweak your raw file. When you close the window, the image gets imported into GIMP.

That does work, and the maintainer is incorporating an info pop-up into most recent builds.

Apologies, I was using Linux when using Rawtherapee.

Looks like it is broken in Windows.

The best I can suggest is use it as before, open image in Rawtherapee, send it to Gimp. (some screenshots: in a Win10 VM)

I used the "other command line" option http://i.imgur.com/SIsXxdV.jpg

Then send to external editor http://i.imgur.com/rORoTNt.jpg

Which opens, eventually, in Gimp 2.9.6 as a 16 bit image. http://i.imgur.com/NWNaqdI.jpg

More trouble than it is worth, apart from being slow, Rawtherapee opens a terminal, similar to Gimp 2.9.6 which you need to close.