If you just added the Asterisk packages using,
sudo add-apt-repository "deb http://packages.asterisk.org/deb `lsb_release -cs` main"
and are now trying to install Asterisk from packages and you get an unsigned key problem,
then you must do as root,
wget http://packages.asterisk.org/keys/175E41DF.pub -O - | sudo apt-key add -
You can now reload the packages within Synaptic.