You could try adding an entry to the firewall like so (untested):
Rule name: anything
IP version: IPv6
Internal IP address: IPv6 address of your host
External IP address: leave blank
Protocol number: 58
Note that this will presumably let through all ICMPv6 packets (not just pings). There's more to ICMP (whether over IPv4 or IPv6) than just pings. Sadly, the GUI firewall interface on ADSL routers tends not to provide fine-grained control, and this router is no exception.