How to add and use a package in package on a Tender- first, a Package in Package must be created and available to select on the tender - see Setup - Package in Package

To start, open an existing Tender or create a new Tender- see How to - create a new tender and How to - editing a tender

Then, find, and select any Package which contains another package- this is the same as selecting a regular package on a tender.

Once a package is selected and changes are saved, any included packages will also be selected or included (this depends on the Package in Package link type - see Setup - Package in Package for more details on Package link types)