problem with custom theme css file

I’m building a custom design for a site and copied the default theme folder, renamed it, and am beginning to modify the css file. The problem is the new theme (which shows up in the Presentation > Themes) isn’t finding the location of the copied CSS file that I’m modifying. The code is identical to what the default was:

<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen" />

What else do I need to do to get the path correct? FYI, I know very little PHP so the more detail anyone can provide me, the better. Thanks.