I downloaded the standalone reference application, start it and login successfully. Unfortunately the concept manage page is not found (HTTP 404). Since the path [http://localhost:8081/openmrs-standalone/owa/conceptdictionary/index.html] contained "owa" I figured it must be the OWA concept management module missing.
I checked the OWA management page by following
"System Administration"->"Advanced Administration"->Open Web apps (Manage Apps) Indeed there was no app installed. I went ahead and downloaded the Concept Dictionary OWA module, however trying to upload it throws an error "App is not a zip archive" with the console saying something else as shown in the screenshot below.
To reproduce the problem: On fresh installation of standalone version 2.6.1 reference application go to `"Configure Metadata" -> "Manage Concept Dictionary".
On examining the server log (see pastebin) it seems the zip file is being up uploaded and unzipped.