To use these modules, add loading lines in /boot/loader.conf:

if_stf_load="YES"

If you have if_stf.ko in /boot/kernel directory, overwrite it by
copying from /boot/modules or remove it from /boot/kernel.

6rd setting is up to your ISP.  E.g. SAKURA Internet in Japan provides
example setting in their site (in Japanese).

http://research.sakura.ad.jp/6rd-trial/6rd-trial-freebsd8/