Have you downloaded Google Play Music? You'll need to download the Music Manager to your computer and then you can sync your itunes collection to Google Play Music. I have a PC so not sure about your Mac. It takes a while to upload if you have a large collection but once it's done everything you add after that will auto sync to Google Play....I believe if you are running Mac OS 10.5 or above you should be able to accomplish this.
Im a 10-month Nexus 7 user and i often use the DoubleTwist for Mac to sync my Nexus 7 with iTunes on my Macbook Pro. Followings are the steps:
1. Separately download and install the DoubleTwist for Mac and DoubleTwist for Android onto your iMac and Nexus 7.
2. Connect your Nexus 7 into your iMac with its USB cable. Once connected, you will be able to see a new storage button on your Mac screen.
3. Run the DoubleTwist app, an instruction will pops up which will explain to you how to sync your iTunes stuff including videos, songs and pictures with your Android device. By the way, for transfer DRM-protected iTunes files, you need to strip DRM from them firstly. This little app can help you do this work.
4. After you fully understand the whole sync process, you can start to do this. It is pretty easy.