Hey @shenandoahu,
1. Have you “Network Activated” the plugin?
2. When you say that the plugin “won’t register on the secondary subsite”, can you please clarify what happens when you try using the key there? If a specific message pops up, can you please share screenshots of that message? You can do so by uploading the screenshots to this thread. If you have issues with that, you can upload to Imgur.com, Flickr.com, CloudUp.com, or any similar image-hosting site; then just share the links to those images here and I’ll take a look.
Thanks!
George