Everyone has an opinion

build and install from ports (/usr/ports/net/miniupnpd)

Add this to your /etc/rc.conf

miniupnpd_enable="YES"

Next add a few things to your /etc/pf.conf

rdr-anchor "miniupnpd"
anchor "miniupnpd"

pass out on $int_if from any to 239.0.0.0/8 keep state
pass in on $int_if from any to 239.0.0.0/8 keep state

Also change your NAT entry to add static-port

nat on $ext_if from $int_if:network to any -> ($ext_if) static-port

I tested from functionality from a couple OSX boxes with the following:

dns-sd -X udp 123 456 789

If upnp is working, it should return an address:

mifune:~ djh29$ dns-sd -X udp 123 456 789

Timestamp     if   External Address     Protocol        Internal Port   External Port   TTL  
10:32:20.589  0    70.90.241.189        16              123             0               0    
^C
mifune:~ djh29$

I fought with this thing for awhile before I realized that the multicast address rules were necessary under FreeBSD (per the INSTALL doc for miniupnpd).