Authenticating a repo?

[Stumpy]

Hi, I wanted to install icecat so I thought that it would be best to add
the repo so that updates would be automc. I went thorough the steps
which workd but it gave me the warning first that the repo was not
authenticated and that it was a bad idea so I tried, and I dont know so
much about authenticating, to download there pgp key,
http://archive.trisquel.info/trisquel/trisquel-archive-signkey.gpg
and move the key over to my templatevm then add it, I am not so sure
about how to add it, I tried
sudo apt-key add trisquel-archive-signkey.gpg
it said ok so I thougth all was good but when I tried to update the
template It told me:

W: GPG error: http://mirror.fsf.org/trisquel toutatis-updates InRelease:
The following signatures were invalid:
E6C27099CA21965B734AEA31B4EFB9F38D8AEBF1
W: The repository 'http://mirror.fsf.org/trisquel toutatis-updates
InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore
potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user
configuration details.

I looked over the man page but its a bit much for my brain. I am
thinking that I did not add it the right way but can someone verify that
and let me know the rite way?
Thx so much.