You can download the MasterClass app for your Roku in one of two ways:
1. Search MasterClass in the channel store within your device (it is called MasterClass - Learn New Skills) to download it to your device.
2. Follow this link to the app listing, make sure you are signed in with your Roku account, and download the app. https://channelstore.roku.com/details/579323/masterclass-learn-new-skills
How Do I Access My MasterClass Account on ROKU?
Once you download the app, you can log in once you launch the app. If you do not have a MasterClass account, you can create one on the first page once the app is launched.