Public Key Error in Downloading FreeNx !!!

[exploder]

I am trying to download FreeNx server from Kanotix and Debian repositories. After apt-get updating I get the following error related to public key:
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Reading package lists... Done
W: GPG error: http://ftp.au.debian.org unstable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D
W: GPG error: http://kanotix.com sid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FB1A399A71409CDF
W: You may want to run apt-get update to correct these problems
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Any suggestions ?

[St. Joe]

Install gnupg (if needed) then repeat steps for any needed keys.

# apt-get install gnupg
# gpg --keyserver wwwkeys.eu.pgp.net --recv-keys F1D53D8C4F368D5D
# gpg --armor --export F1D53D8C4F368D5D | sudo apt-key add -
# gpg --export F1D53D8C4F368D5D >> /etc/apt/trusted.gpg
# gpg --list-keys

[exploder]

Thank you Saint. You are a life saver. I really appreciate your help. It worked. But, now I have some other problems. I will fix it.