I just found out that Roller Coaster Tycoon 3: Platinum was added to Steam. I bought the game years ago, and unfortunately lost the disk. Is there any way to add RCT3 to Steam w/o the disk?
Roller Coaster Tycoon 3 cannot be added to steam through the activation key. It must be purchased through Steam to activate it.
For a list of all the games that can be activated on steam, see here:
If you want to put the game on your Game library as a Non Steam Game, then follow these steps:
Launch Steam. Click the Games menu, choose Add a Non-Steam Game to My Library. Browse for games on your computer or put a check next to the game(s) you wish to add to the Library. Click Add Selected Programs