I have the latest Extensions API Developer Preview installed with the npm web service running. When opening up Desktop, the extensions option shows up under objects but when dragging it over to the Dashboard, the dialog opens but just seems to stall without showing the list of available .trex files. Tableau at this point stalls and need to be closed and reopened. I also disabled accelerated graphics option under the Settings and Performance menu which did not help. Not sure what I could be missing.
Thank you for participating in the Developer Preview for the Extensions API! You'll want to post this feedback in the PreRelease site where you accessed the Extensions API - the developers building the API are active there, taking feedback and troubleshooting issues like this one.