It looks like you're missing the listenerDataType attribute. The following example works for us albeit using Enterprise Library 5.
<add listenerDataType="Microsoft.Practices.EnterpriseLibrary.Logging.Configuration.SystemDiagnosticsTraceListenerData, Microsoft.Practices.EnterpriseLibrary.Logging, Version=5.0.414.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
type="Microsoft.WindowsAzure.Diagnostics.DiagnosticMonitorTraceListener, Microsoft.WindowsAzure.Diagnostics, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
name="LogListener" />