As the time of writing this post, Viber didn’t release an updated version that supports Mac OS X Yosemite.
Chances are the app icon is marked with an X and you get an error message once you try to launch it.
It’s pretty easy to actually fix this issue; Just head to your Applications directory, right-click on the Viber icon then Show package contents. Navigate to the Contents folder and open Info.plist ; Note that you’d probably need a Plist file editor installed (or XCode) to edit this file. If it’s not the case, just open it with your preferred text editor – and you’ll get something like this:
Notice the highlighted lines – all you need to do is change the values 4 and 4.0.0 to, respectively, 5 and 5.0.0
Save your file. Relaunch Viber. You’re good to go!