This is an issue at HEAD (verified on this install)

You must be in at least 1 room already.
Go to https://secure.phabricator.com/conpherence/
Click on Participants->Edit Room
Observe nothing happening.
Open the console and see: core.pkg.js:216 POST https://secure.phabricator.com/conpherence/update/null/ 404 (Not Found)

At this point if you click on other rooms and go back to the original room then go to edit the room, the dialog will come up, but for the previous room that you were in.