1.) Run this command in a terminal window to add the repository GPG key:
wget -q -O- http://archive.getdeb.net/getdeb-archive.key sudo apt-key add -
2.) Open Synaptic Package Manager, then click to Settings -> Repositories. In Other Software tab, click Add the GetDeb source for Natty:
deb http://archive.getdeb.net/ubuntu natty-getdeb apps