Ahhh, you're talking about the Roku.com web page. Not the Roku device's Developer's Web Page.
You upload a zip file into your own Roku device. Then download a Pxxxxxx.pkg file from your own roku device.
That .pkg file is what you upload/submit to Roku.com as the Package.
The .pkg file is created locally on your own Roku device using a web browser on your computer.
Try reading two sections of the
Channel Packaging and Publishing guide:
Section 3.0 Packaging Applications
Section 4.0 Managing Channels