This and much more is included in the README that comes with the plugin.

Initially (after installing the plugin, and renaming the plugin directory from auto-admin to auto_admin (required for Rails 1.2.1) obviously), you need to add a few lines to the bottom of your environment.rb:

  AutoAdmin.config do |admin|
    # This information is used by the theme to construct a useful
    # header; the first parameter is the full URL of the main site, the
    # second is the displayed name of the site, and the third (optional)
    # parameter is the title for the administration site.
    admin.set_site_info 'http://www.example.tld/', 'example.tld'

    # "Primary Objects" are those for which lists should be directly
    # accessible from the home page.
    admin.primary_objects = %w(actor film user)

    admin.theme = :django # Optional; this is the default.
  end

Having done that, you can now (re-)start script/server, and navigate to http://localhost:3000/admin/. Yes, it installs its own routes. Yes, they're hard-coded. Yes, that needs to change... for now, just don't try to use /admin/ for anything else. :)