When I run sudo apt-get update I have the following output:
Hit http://mirror.transact.net.au stable Release.gpg
Ign http://mirror.transact.net.au/debian/ stable/contrib Translation-en
Ign http://mirror.transact.net.au/debian/ stable/contrib Translation-en_AU
Ign http://mirror.transact.net.au/debian/ stable/main Translation-en
Ign http://mirror.transact.net.au/debian/ stable/main Translation-en_AU
Ign http://mirror.transact.net.au/debian/ stable/non-free Translation-en
Ign http://mirror.transact.net.au/debian/ stable/non-free Translation-en_AU
Hit http://mirror.transact.net.au stable Release
Hit http://mirror.transact.net.au stable/main amd64 Packages
Hit http://mirror.transact.net.au stable/contrib amd64 Packages
Hit http://mirror.transact.net.au stable/non-free amd64 Packages
Get:1 http://www.debian-multimedia.org squeeze Release.gpg [198 B]
Ign http://www.debian-multimedia.org/ squeeze/main Translation-en
Ign http://www.debian-multimedia.org/ squeeze/main Translation-en_AU
Ign http://www.debian-multimedia.org/ squeeze/non-free Translation-en
Ign http://www.debian-multimedia.org/ squeeze/non-free Translation-en_AU
Get:2 http://www.debian-multimedia.org squeeze Release [22.0 kB]
Ign http://www.debian-multimedia.org squeeze Release
Get:3 http://www.debian-multimedia.org squeeze/main amd64 Packages [72.8 kB]
Get:4 http://www.debian-multimedia.org squeeze/non-free amd64 Packages [3,902 B]
Fetched 98.9 kB in 3s (32.6 kB/s)
Reading package lists... Done
W: GPG error: http://www.debian-multimedia.org squeeze Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 07DC563D1F41B907
I've scoured the web but can only find esoteric solutions I cannot understand.
Is there a way to bypass the security feature blocking it without engaging in esoteric commandline?