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 copy an existing Package to create a new Package via the Clone feature.
You can select Products in the Product Explorer and use the Add Items To button to add them to a Package, Allowance, or Bundled Cable.
You can select Labor Items in the Product Explorer and use the Add Items To button to add them to a Package or Allowance.
You can create Packages within the Project Editor by selecting items and using the Add Selected Items To feature. The Packages will be Project-specific unless you decide to use the Update to Catalog feature.