To login via SharePoint Designer:

• Open SharePoint Designer.
• Click on "File" on the top left of the screen.
• Click on "Open Site".
• In the "Site name" box, put in the URL to your SharePoint site (ex: http://www.yoursharepointsite.com)
• Click on "Open".
• Under "User name:", please put in your main SharePoint user name.
• Under "Password:", please put in your password.
• Click on "Remember password" if you'd like to save it for future use.
• Click on "Ok".

At this point, you should be logged into your SharePoint site and be ready to edit it using SharePoint Designer.