AH! Could be because the links to the articles are generated in the community, which often requires a login.
They should be redirected to this:
https://community.marketo.com/MarketoArticle?id=kA050000000L9EwCAK
As you can see from the instructions, it doesn't require a login. If you screen shot those instructions then send them out to those who you've issued licenses to, that should help! Also, you should post this as an idea! (IE Instructions for installing the add-in that are hosted on a site outside of the community so they can access them) 🙂