Invitations have been sent out to select users to take part in a private beta test. If your one of the lucky ones selected you should get an email inviting you to take part , but you'll need an Apple device (iOS 6 or later). If so, please share the IPA file to others here. Remember, share is care :-)
Source: http://www.3g.co.uk/PR/August2013/blackberry-messenger-for-iphone-and-android-enters-testing.html