1, First step is to buy and download the game from GOG.com. It cost $9.99. (http://www.gog.com/game/rollercoaster_tycoon_2)
2, Then you have to download a wrapper for free here: (http://paulthetall.com/rollercoaster-tycoon-2-mac/)
3, Next thing is to make sure that you have the "setup_rollercoaster_tycoon2_2.0.0.6.exe" (from step 1) file and the "Rollercoaster Tycoon 2.dmg" (from step 2) file on your desktop.
4, Open "Rollercoaster Tycoon 2.dmg" and follow the instructions in this video.
It begins at 3:03.
Don't worry about the small game window in this video. They say it's only a bug while recording. I play fullscreen without any problems.
If you or anyone else have any problems. Feel free to ask and i'll try to help the best i can.