Just uploaded s2member and am trying to figure out how the administrator can add username/passwords as opposed to the user gaining access by just filling out the registration page. Is there a way that users cannot register but will only be able to login with a username/password that the administrator creates?
Yep, Mekipstrob is right. WP Admin -> s2Member -> Add A Member or WP Admin -> Users -> Add New
And if you don't want people to register on their own, disable free registrations. WP Admin -> s2Member -> General Options -> Open Registration / Free Subscribers
There is another free plugin that lets you create login and logout links in the sidebar: “Login Logout” - just search for it in the Plugin panel in the WP Dashboard. You can set it to appear on all pages (protected or not) as the redirct will be to the member login page where they either can login or, if they can't, will be redirected further to the membership options page.