Yes you can, no matter if you don’t have a 3G version and here is a quick guide how to do it.
To activate Viber on your iPad (or iPod Touch), you can use the number of your cellphone. After you have received an SMS with an activation code in it, just enter it into your iPad or iPod Touch and that’s it.
Viber was primarily developed for iPhones, therefore, to achieve maximum experience, it is recommended to use an iPhone with iOS 4. Even though Viber is known to work on iPod Touch and iPads, they are not officially supported.