I'm also having this issue, but it seems to be simply a matter of where to put the ArcRest files. I have an ArcREST folder within the Python27\ArcGIS10.3 directory, and clearly some of the ArcRest python files and modules are not where they need to be.
Any thoughts on where to put the ArcREST files once they are downloaded? I have run an install using pip for ArcRest, and it ran successfully but again, not everything is apparently stored where it needs to be.
Thanks -
Allen