sixxs.net does not exist anymore. You want to establish an IPv6 connection to your provider using DSL?

Use FortiOS 5.4.6 or 5.6.2.

Configure your interface:

config system interface
        edit "wan1"
        set mode pppoe
        set allowaccess ping
        set type physical
        set role wan
        config ipv6
            set ip6-mode pppoe
            set ip6-allowaccess ping
            set dhcp6-prefix-delegation enable
            set dhcp6-prefix-hint 2001:db8::/48
            set dhcp6-prefix-hint-plt 900
            set dhcp6-prefix-hint-vlt 10800
            set autoconf enable
        end
        set username "<your username>"
        set password ENC <your password>
        set dns-server-override disable
    next
end

The important parameters are:

config ipv6
     set ip6-mode pppoe
     set dhcp6-prefix-delegation enable
     set autoconf enable
next

Maybe your provider requires other settings. This method has been tested and works in our environment for several months now.