I am a Brand Spanking New Gentoo Community Member... and I just figured out how to "unmask" packages.
You have to do an emerge like so:
...as well as have it added to "/etc/portage/packages.keywords".
You can add it by typing this into the command line:
echo "=app-cdr/nero-220.127.116.11" >> /etc/portage/packages.keywords
Hope this helps. I had a B* of a time figuring out how to unmask packages.
I <3 NeroLinux3... it rawks!