Packages are made available through so called repositories. The repository contains the packages and an index which includes some basic information about the packages, e.g. required dependencies or a short description.
Repositories can be located on many medias: CD-Rom/DVD, local hard disk, the Web (http,ftp) or remote file system (nfs).
Choose Edit->Refresh Package List from the menu.
Click on Update List in the toolbar.
You can edit, add or delete repositories in the repository preferences.
Open the repository editor with Preferences->Repositories from the menu.