Hello Michael,
You are running ip6tables v1.3.5 but MSS clamping wasn't added until
1.3.8. Do a search for "mss" in the changelog and you'll find it:
http://ftp.netfilter.org/pub/iptables/changes-iptables-1.3.8.txt
The current stable version of iptables and ip6tables is 1.4.11.
--David