Are you sure the plugin is activated? The plugin consists of only one PHP file, and the previous_post_link_plus function is defined at line 204. I have heard of people getting an undefined function notice after they uninstall the plugin, but never while it is activated.
I know the plugin names are similar, but I am using the same terms (“post” and “page”) that WordPress uses. I think if I were to create my own terminology to refer to posts and pages, it would make things even more confusing. The next time I update the plugins, I’ll put a big bold notice at the top of the documentation alerting people to the difference, and to make sure they are using the right one.