Try adding the section posted below to your /etc/apt/sources.list and run apt-get update. The first two commented out lines are the commands to get the GPG key. I usually comment out the archive line after I get kernel stuff I need from unstable, or else it may be upgraded often from snapshots.
Code:
##################################
##### Kernel Archive Builder #####
##################################
#gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 29F335B3
#gpg --armor --export 29F335B3 | apt-key add -
deb http://kernel-archive.buildserver.net/debian-kernel/ trunk main