You must import the IFrame in the <mx:Script> too. The namespace only works for mxml.
import com.google.code.flexiframe.IFrame;
On your screenshot, you just add to hit enter to have it done ;)
One thing: I see you use Flex 4, but flex-iframe is not yet compatible with Flex 4... See
http://code.google.com/p/flex-iframe/issues/detail?id=42