How do I set up IPv6 on my Airport Extreme?
In the "Configure IPv6" menu, select "Automatic".
In the "IPv6 Mode" menu, choose "Native".


That should be all that's needed. The device should be able to autodetect the rest of its settings.

If that doesn't work, then you may need to check the settings with your ISP.

Source / more info: http://support.apple.com/kb/PH5098