In order to check that the stack works, execute #ping6 ::1

If the result is the following (or similar):

PING ::1(::1) with 56 data bytes

64 bytes from ::1: icmp_seq=1 ttl=64 time=0.046 ms
64 bytes from ::1: icmp_seq=2 ttl=64 time=0.016 ms
64 bytes from ::1: icmp_seq=3 ttl=64 time=0.013 ms
64 bytes from ::1: icmp_seq=4 ttl=64 time=0.012 ms
64 bytes from ::1: icmp_seq=5 ttl=64 time=0.009 ms

It means that you already have installed, correctly, the IPv6 stack in your machine.

The only thing you must do is publishing the file /etc/rc.conf and adding the following code line:

IPv6_enable="YES"

Moreover, if you want to assign an IPv6 fixed address to your machine, you also have to add a code line similar to the following:

ipv6_ifconfig_r10="2001:618:10:4:4 prefixlen 64"

Where 2001:618:10:4:4 is the IPv6 address and 64 is the network prefix.