The formatter isn't really intended to be used directly, so there's no explicit factory for it. May I ask why you want to get the formatter (the scenario could be useful to us for future planning).
In Entlib 5, you should be able to do something like:
var formatter = EnterpriseLibraryContainer.Current.GetInstance<ILogFormatter>(formatterName);
But, as I mentioned, as it's not a public entry point this isn't guaranteed to work forever.