Assuming you've already constructed another configuration source (FileConfigurationSource for example), then the easiest way to get a container to pick up the registrations would be:
var container = new UnityContainer()
That will give you a container that's configured with Entlib out of whatever config source you're using. If you leave out the parameter, it'll use the default app.config.
You can also then load a Unity configuration section into the same container to pick up whatever other registrations you might need, or use the Unity API or whatever.