Google Product Forums
Re: Your pages are displaying blank ad units because you haven't yet verified your PIN and/or phone number.
May 25, 2012 3:20 AM
Posted in group:
AdSense Help Forum
I have an active account: Working with AdSense
You should have received a PIN on a postcard in the mail, at some point.
If it didn't arrive, or if you lost it, you can request another one.
Here's the troubleshooting page: