As ipv6 package is not part of basic bundle, you may have to install it first - download "extra packages" from download.mikrotik.com (be sure to select correct CPU architecture, probably it's mmips, and version which is exactly the same as the one running on your router), extract the ipv6-*.npk, upload it to router (to non-volatile storage, could be under /flash) and reboot the router.
After you enable/install the package, it will come without any configuration what so ever ... that includes firewall filter. The best approach at populating it is to list default configuration using /system default-configuration print
... but that only works in command line. Be sure to make terminal as wide as possible, wide config lines don't get wrapped, they are truncated. Scroll down until you reach /ipv6 firewall
and copy-paste the whole section:
(ends with closing curly brace). On my router (running ROS 6.47.1) section which follows ipv6 is [/i]/ip neighbor discovery-settings[/i]. Run the copied section on command line which should set IPv6 firewall with default settings. The rest (address, ...) is then up to you ... using GUI. Some potential firewall changes can be done via GUI as well.