Google posted a notice in the Google AdSense Help forums that those developers new to AdMob with unapproved AdSense accounts will currently not be able to link the two together.
Google is working on a fix for this but didn't give an estimate time of release.
Google's AdSensePro said:
Many of you have upgraded your AdMob accounts to the new AdMob (apps.admob.com) and are wondering why you can't use the associated AdSense account to monetize websites. At this time this functionality is only available to users who had a fully approved AdSense account prior to upgrading to new AdMob.
We appreciate your patience and feedback as our engineers work to add this functionality for all users.
Forum discussion at Google AdSense Help.