It sounds like a few folks are getting stuck on some parts. I've attempted a more streamlined tutorial below, which has worked for me numerous times. I hope you find it helpful!
Here we go:
http://openrct2.org/downloads Install OpenRCT2 for Mac.
Unpack the zip file. An application will appear, move into your "Applications" folder. I like to just rename it RCT2.
http://store.steampowered.com/ Install Steam (green button at the top of the page).
After you've installed the app and have created a profile, purchase RCT in the app.
You'll need to open a "Console" tab in Steam that isn't normally there. To do this, search for the "Terminal" application on your Mac. Or, you can easily find it by going to the Applications Folder in a new Finder window- it will be inside the "Utilities" subfolder. Once you have Terminal open, copy and paste:
/Applications/Steam.app/Contents/MacOS/steam_osx -console
Typically you don't need to press enter... once you paste, it should do it's thing- it will run the Steam App again, and this time you'll see "Console" as the fifth tab at the top, after your username.
In the Console tab, enter:
download_depot 285330 285331
When the download is complete, you should get something like this:
Depot download complete : "/Users/<username>/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS\steamapps\content\app_285330\depot_285331" (2525 files, manifest 6046146891100703072)
You need the section within the quotation marks. Notice how after "MacOS" the slashes change from "/" to "\".
Click over to your desktop, so the top nav bar on your screen says "Finder". Select "Go", then "Go to Folder" from the drop down menu. Paste your file path that you received from Console, while also changing all your "\"s to "/"s. In this case, you would paste:
/Users/<username>/Library/Application Support/Steam/Steam.AppBundle/Steam/Contents/MacOS/steamapps/content/app_285330/depot_285331
Your finder window should have pulled up a folder called "depot_285331". It's about 1.1 GB and has a bunch of folders in it. Copy the entire folder, and paste it into your Applications folder. I like to rename mine RCT2 to match the app from Step 2.
Run your RCT2 app! The game will ask you where it's supposed to find the files for you to play this game. In the dialog box, direct it to Applications > RCT2 (folder).
