You can download it right now, but the app is an Early Access release and reviews are, to say the least, mixed.
Some users are reporting dropped frames despite powerful PC setups, and (many) others are saying that it doesn’t work at all, crashing on startup. At the same time, some folks affirm that it works just fine for them, and one user even said that the 360-degree videos play on an Oculus Rift, even though the app is ostensibly for HTC’s Vive.
Problems are normal for a beta release, but YouTube VR seems to have a major bug that stops the app completely for many. Hopefully, it will post a new version soon, or some enterprising Steam user will find a decent workaround.