Login-Button in Plugin

Julian,
Unfortunately, I don't know the exact answer to how that template or the plugins that come with it work. My guess is that the Login/Register area is a module that they have and you should be able to edit it's template to add the Login With Facebook button how you see fit. I can see doing this by adding it in between the buttons, like you suggest, or by adding it individually to the Login and Register drop-downs that appear when you click them.

Looking at your template, it seems like "JA Quickaccess" is something you'd want to look for. Likely, it's a module, not a plugin, that's showing there and it's in the "quickaccess" module position, from what I can tell.

Once you find it, try editing the template (likely default.php) for that module. Then, you can add the {JFBCLogin size=medium} tag (or size=small) and see how that works out. If you find the right file to add it too, we can gladly help you further if you run into issues. You may need to contact JoomlaArt to learn more about their template, and how to modify it though.. that's outside our area of expertise.

Hope that helps,
Alex