$theme_path = base_path() . drupal_get_path('theme', 'my_art_drupal_theme');
$css_path = $theme_path . 'css/MyFontsWebfontsKit.css';
drupal_add_css($css_path);
The fonts may still not show up but check the view source of the rendered page to see if the stylesheet was included. The paths to the font files may need to be adjusted in the stylesheet.
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Change the folder and file name.