Note that you may hav to disable Conversation for your Google accounts in order to be able to set up iMessage.
Mac keeps the conversation going. Messages does everything iChat does, and so much more. For starters, it comes with iMessage. And just like iMessage in iOS, it lets you send unlimited messages to anyone on a Mac or an iPhone, iPad, or iPod touch running iOS 5. Send photos, videos, documents, and contacts — even send messages to a group. You can see when your message has been delivered and when someone’s typing a reply. Turn on read receipts, and they’ll see when you’ve read a message. With end-to-end encryption, your messages stay safe and private. And you can start a conversation on your Mac and pick it up on your iPhone, iPad, or iPod touch. So nothing is left unsaid.