Missing key issue

[John W Foster <jfoster81747@verizon.net>]

Anyone know how to get rid of this issue. I,m getting this message with
every reload of Synaptic.

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

Thanks
John