The Bisigi themes are not yet available in a repository for Ubuntu Natty Narwhal 11.04 so if you follow the instructions on their site, they won’t appear in the list of products to add. I wrote about this cool set of themes before.
I found instructions on another page, however that showed how to get the update to work. Follow the first step to install the repository:
sudo add-apt-repository ppa:bisigi/ppa && sudo apt-get update
then go into your Synaptics Package Manager, click on the Settings menu and select Repositories. A dialog box with multiple tabs opens up and you should select the second tab, Other Software. Look for the repositories you just added for Bisigi (http://ppa.launchpad.net…) and click on the first one (of two). Then select the EDIT button and change the distribution entry in the dialog that opens from natty to maverick. Click OK and repeat the same step for the second repository.
Now continue the installation steps and the themes will be downloaded and available through your normal display options.