I cannot include a file in my plugin settings page

Most likely the file cannot be found, I’m assuming the file you’re trying to include is located in your theme folder, in that case you should use:

require_once( TEMPLATEPATH . '/file.php');

TEMPLATEPATH will return something like this: /home/user/httdocs/wp-content/themes/twentyeleven.

Worst case scenario you’ll find a Fatal Error, but that’ll help to find the problem.