How do you install custom rides and scenarios?

On Windows 7, you can add tracks by pasting them into the folder via Local Disk ( C: ) -> Program Files (x86) -> Hasbro Interactive -> RollerCoaster Tycoon -> Tracks.
For scenarios, the steps are the same except they're in the Scenarios folder, which should be right above the Tracks folder.

For other Windows operating systems, the location of the Hasbro Interactive folder might be somewhere slightly different, but once you find that one, it shouldn't be any different from there.