Public key is not available

Hi,

I'm trying to install a kernel for my headless power pc box that run
Debian GNU/Linux Wheezy.

I'm trying to do the following steps:
sudo su
echo "deb http://www.emdebian.org/debian/ wheezy main" > \
/etc/apt/sources.list.d/emdebian.list
apt-get update
apt-get install gcc-4.4-powerpc-linux-gnu
apt-get install build-essential devscripts
apt-get install device-tree-compiler uboot-mkimage
apt-get install libncurses5-dev

dget -xu http://b3.update.excito.org/pool/main/l/linux/linux_2.6.39.4-13.dsc

cd linux-2.6.39.4
make menuconfig

At the step:
apt-get update

I get the following Warning:

W: GPG error: http://www.emdebian.org wheezy Release: The following
signatures couldn't be verified because the public key is not
available: NO_PUBKEY B5B7720097BB3B58

How can I solve this problem?

-- 
Regards from Pal