In order to enable this backend by default, you will need to change the file
%%PREFIX%%/etc/PackageKit/PackageKit.conf, replacing the line:

< DefaultBackend=ports

with:

> DefaultBackend=pkgng

Otherwise, PackageKit will default to using the ports backend unless you
explicitly launch packagekitd with a different backend.

To use PackageKit, don't forget to enable dbus (for example, by adding
dbus_enable="yes" to /etc/rc.conf, or `service dbus start`).