Plugin worked perfectly, though documentation is lacking. To actually USE the plugin, I just added this function where I wanted the subscribe button to show up:
And the class of the link that's generated is: "wp-subcribe-author-url" so you can style it with that. NOTE THE TYPO! It's "subcribe" not subscribe.
The only feature that I think would be helpful is to have something show up asking visitors who aren't logged in to register or sign up to subscribe. I guess I will have to make that on my own!