There are two ways that you can add Packages to your Catalog within the Package Explorer.
You can add a Packages "from scratch" via the [New] button. You can choose to create a Package, Allowance, or Bundled Cable, all of which are types of "packages" in SI. The links above will explain the fields for each type of Package.
You can create Packages within the Project Editor as well. The Packages will be Project-specific unless you decide to use the Update to Catalog feature. Click here details.