Adding a login link
From Georgetown University Commons Knowledge Base
Depending on the theme you choose or modifications that have been made to the sidebar, the 'site admin', 'options,' or 'login' link might not appear on the public facing side of your blog allowing you to navigate to the administrative back end. You can easily add such a link to your sidebar.
First, enter your dashboard by adding "wp-admin/" to your current URL and clicking "enter."
Log in, click on the drop down button of the "Appearance" tab on the toolbar on the left, then select the "Widgets" option.
On the "Widgets" page, you will be presented with a list of available widgets that you can add to your blog.
Click on the "Meta" widget and drag it over to the "Sidebar" box on the right, and click save. You should now have a variety of administrative links, including site admin, in your blog's sidebar.