Hi again. I was able to complete the process and the product is now registered.
I did not receive the email, but with some help from an autoreply to my Humble Bundle request, I discovered that in the “Steam” section on the Humble Bundle download page (just above the download section), there is a button labeled “Reveal your Steam key”. That is the key/code that is needed to redeem HackNet in the Steam client.
Thanks again for offering the program. I’m looking forward to playing around with it!