Make sure you turn on your Rift before you start UE4. Then try to play your game by clicking “VR preview”, that should work. In standalone mode you indeed need to go full screen or now that only a direct mode is available, you need to redirect output to the Rift. If Alt+Enter doesn’t work then try executing console command “stereo hmd”, that should redirect output to the Rift, you can also try to set full screen with a command “r.FullScreenMode 0”, see the attached pic (do it in level blueprint). Hope it helps. 
Unreal Engine 4, Oculus 0.6.0.1 commands (you need a GitHub account):
https://github.com/Oculus-VR/UnrealEngine/blob/4.7-0.6/UE4-Oculus.txt
Note: if nothing works, try stopping Oculus runtime and starting it again or restart your PC. Make sure you have the latest gpu drivers and Oculus runtime 0.7 installed.