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

3.) Close the software source window, then click Reload in Synaptic Package Manager window.

Now, the packages in GetDeb.net for Ubuntu 11.04 Natty is available in Ubuntu Software Center.