As of now BlackBerry Q10 doesn't supports Viber VoIP application as this phone is running on latest BB 10 OS and this platfor has poor support for apps, I mean there are very less number of apps available in appworld to phones runs on this new OS platform.
However, the developer of Viber may release the app soon and I suggest you to try sideloading of the ported app on this device so you will get a quick access to this device. This doesn't mean that every features should work perfectly but main features will work for sure.
Moreover, only Google Talk app is available for this platform as of now to connect to the cross platform devices/users rather than BBM for same platform users. Keep an eye on this link of appworld for the development and release fo this app: http://appworld.blackberry.com/webstore/content/85455/?countrycode=US