It's a Roku. If you want to watch an app, you should add the app to your Roku account. When you do, it will show up on your Roku device.
Do you see the Roku main menu when you turn the TV input to the HDMI to which the Roku is connected?
If you do see the Roku main menu, did you look at all the items on the menu? It scrolls all the way around, so what you see on the screen may not be everything until you scroll up or down.
If you haven't added Paramount+ to your Roku account, you should go to the Streaming Channels selection on the menu and look for it there. You can do a text search, or browse; I find it under Movies & TV, Recommended, New & Updated, and in other places. You should see it there too, if you're in the U.S.
Roku Community Streaming Expert
Note: I am not a Roku employee.
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