Have you installed a plugin that will gove you a login widget to place on your site?
Examples:Sidebar LoginAJAX Login Widget++
If so, have you placed the widget in one of your sidebars?
Just enabling registration doesn't automatically place a login/register form somewhere random on your site. You actually have to place links or a widget to give people access. The "meta" widget that comes with Wordpress will provide the links, but to have a form display somewhere, you have to install a plugin like the ones listed above.