If you don't have the key, it should still install in terms of "apt-get install", just give you a warning about unknown key.
I would open a terminal and do
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 274b5881e00011fb79e2eae15f99a088342d17ac
to import the key. Then
should check the repositories and let you know if the key is working.