First you need to update the Playbook Your Blackberry Playbook comes You will have to wait as Developers are the only ones with access to Google Play. Once the new update is released for your Playbook, Blackberry will allow you to browse Google Play and download Android Apps.
To get Google play on your blackberry phone, you first need to install the app on the phone. After buying it from the app market, sign in using your Google user name and password. You will then get the notification message that the app has successfully been installed.