Version Info this help item shows Joomla v1.0 - almost identical in v1.5

Login at http://www.YOURSITE.NAME/administrator/ using your username and password. Add this page to your Favorites or Bookmarks to make it easy to return.

Help Tip
If that url doesn't work, try
http://www.YOURSITE.NAME/administrator/index.php?option=com_login

Now you should be at the opening admin page called "Control Panel". You can get back to this page by clicking "Home" in the top site menu which is right below the black Joomla Administrator image.