Hello @tomokikato8, I believe you are asking about how to get started after purchasing Medium. To install, go to the Medium page here in Skyline - you can also log in to Oculus.com and go here: https://www.oculus.com/experiences/rift/1336762299669605/ - and press "Install."
Once you've installed, press "Start" and you will be able to start with new Medium tutorials. Let me know if you encounter any more troubles.