If a UA Revised ranger uses Primeval Awareness (pg. 4), are they able to detect enemies in the ethereal plane?
Yes, if the Ranger is in the Ethereal Plane
There's nothing stopping the feature from working in the Ethereal Plane, so of course it does.
No, if the Ranger isn’t in the Ethereal Plane
The ethereal plane is a different plane of existence from the material plane, it doesn't really make a lot of sene to talk about ‘distance’ from the Ethereal plane to the Material plane, so an enemy on the Ethereal plane isn’t "within 5 miles" of the Ranger.
See also this answer about whether or not you can Misty Step between the Ethereal plane and Material plane for a bit more discussion on that point: