I have the Game RollerCoaster Tycoon for Windows/PC. I was wondering that now I have a Mac, if I can somehow take that game and put it on Steam, and play on my Mac, SO.. I went on Steam and went to "ADD A GAME" then "ACTIVATE A PRODUCT ON STEAM" because RollerCoaster Tycoon is on Steam, then I went to the box and couldn't find an Activation Code? Is there one? Or Where do I get one? Or am I going to need to buy the game over again?
The only Roller Coaster Tycoon game on Steam is Roller Coaster Tycoon 3 so I will assume that is the game in question.
Per this list you cannot activate Roller Coaster Tycoon 3 via a CD key. You will need to purchase it from Steam. I would suggest waiting for the next time it is on sale, since that will likely sell it at 50-75% off.
Based on your OS, you might be able to run the game disc using Boot Camp.