If you have installed the Beta version of Avira Free Android Security you need to update the app to the latest version 1.0 to be able to use the full functionality of Avira Free Android Security, for example Wipe data.

If you don’t have the automatic update activated, you have to go to My Apps in Google Play or just open the notification bar at the top of your screen.
There you can see the applications for which an update is available. Just click on the name of the app and perform the update.

Note:
You don't have to re-register again. All your registration data will be stored in the app.
After updating to the latest version you'll get a prompt for enabling adminstration mode.