There's a reason the router found the tunnel -- it is built into the IPv6 spec. It uses a reserved "anycast" IPv4 address to forward traffic through.
When I saw the "auto-config" button on the router setup page, I pressed it half-thinking "this'll never work" and was absolutely amazed when it did (do these "auto" buttons ever work?!) It seems that the boffins behind IPv6 thought through many of the deployment issues.
Why repeat what better writers have already created...
- Human readable explanation: http://en.wikipedia.org/wiki/6to4
- RFC3068: An Anycast Prefix for 6to4 Relay Routers http://www.ietf.org/rfc/rfc3068.txt
The Wikipedia article also contains a link to public 6to4 relay points, however the published link has been updated and indicates that it is no longer needed since RFC3068 exists.