Rating
Nobody has played it on Walnut yet, so it has no medal. Scripted benchmarks have run it (Test runs), but they never rate a game.
Installation
With the buttons above: Install, then Add game files... and pick your copy of RollerCoaster Tycoon 2: the disc in the drive, an ISO, BIN/CUE or MDF image, an installer or a folder. The game data is not part of the package, and Walnut does not offer it.
What works: RollerCoaster Tycoon 2: its CD, an installed copy (Steam, GOG) or the GOG installer.
Optional, from a copy that has them: the expansion: Also add in the app.
More ways (a terminal, game-data-packager on any Debian system, copying the files by hand): Guides.
The game's home page: https://openrct2.io/. License: GPL-3.0-or-later engine, commercial game data.