I’m not sure what you mean? The Menu bar has ‘Username’ and ‘Password’ edit boxes and then the Login and Forgot Password buttons. That entire block should be centered.
If it is not, something within the theme you are running is overriding some of my CSS.
Please try to explain what you are trying to do. If possible, provide screenshots or links to the site in question too, so I can look at it and see what’s going wrong.
Lastly, just want to mention that I am working on an update for this plugin, and one of the new things will be filters which will allow custom theme developers to override the display of various parts of the plugin. This may make it easier to make this change as well (although I’m not clear on your question).