Usually, activating Viber does not present a problem. However, in some cases, more specifically, in certain countries and in case of certain mobile operators, the activation can somehow get stuck.
During the installation process, you should get your access code by SMS. If you don’t receive SMS within 1 minute, you can try pressing the “No code?” button which will guide you through an automatic call procedure. Accept the call, write down the access code and enter it into Viber.
If the automatic call does not work (did not manage to get through), try uninstalling Viber and wait for 24 hours. After that period, try installing it again from the begining.
If even that does not work, you can post your problem at Viber Support. There is a wizard which will guide you through the process of describing the problem. Finish the wizard and support team will contact you in a few days.