then when you click on the zipfile winrar will open it in it's program with all the file contents.
you'll wanna make a nice brand new crisp empty folder for your game, don't put anything in there. name it after your game so you can find it.
then go back to the winrar program and click the button on the left top hand side and it says something like "Extract To"
Then another window will pop up asking you where you want to extract the file contents to. that is when this new folder comes in handy. click on that empty folder named after your game that you just made.
then click okay or enter or open or whatever it says and it'll extract the files and you will be good to go.
then you just gotta find that file you just made and extracted the zipfolder to. and your game should be in there along with a few other random folders and read me's
it will be clear which ones the game because it will have some kind of icon that represents the game. click on that and your game should load you fat peice of shit.
OS X should be able to unzip files on its own. Double click on the zip file. A little window should open that says "Unarchiving" and it'll show the progress. It'll create an unzipped folder of the same name as the zip file wherever you have the zip file stored. If this isn't working try stuffit expander like I said earlier.