Formating your sd for the raspberry pi 2.
I went through a various number of programs to rewrite over the sd card. The one that worked for me was the WinDisk32 imager. But for Mac Apple Pi baker is going to be your best bet.
First you are required to get the image file from the retropie website. From there you want to make sure you have your sd card formatted to fat32.
From their you can use the image you have downloaded and rewrite the sd card with the image rewriting tool.