This is how to put Viber onto a nexus 7:
1. First go to your settings on the nexus 7. You need to make sure that Google Playstore is NOT the default for downloading Apps. You need to UNCHECK the box.
2. Again in Settings, in Security, make sure you ALLOW downloads from outside the Google Playstore
3. Go to google and download Viber.apk
4. Reread number 3 - you must downlaod viber.apk
5. Once this has downloaded, follow instructions. You will need a phone number to link with the account. you will get a four digit number and this allows viber to work.
This really worked for me.
Good luck and please let me know of this was helpful. You CAN get viber to work on a Nexus 7!