NuxLi To enable lightdm, add lightdm and remove slim from the DAEMONS line in /etc/rc.conf.
Mine looks like
DAEMONS="sysklogd dbus lightdm alsa networkmanager"
Then you need a file in /usr/share/xsessions for lightdm to find xfce, there probably is one already, but if not, create a file called somethinng like xfce.desktop with the contents
Then you should be able to choose Xfce from the sessions menu in lightdm.