This is a member of the Viber Development Team.
Viber application for Android is still at its Beta stage. We are still not sure when exactly it will be release officially.
However, you can subscribe to be beta testers here: Android Users and Testers
Once you subscribe, you need to wait (several weeks usually) for our invitation email, after which you'll be able to complete the installation and use the app on your device.
If you have more questions - feel free