one of two things is causing the issue:
1) The code modifications you made are causing the session not to be set.
2) The server is not configured to maintain session state properly.
to rule out the server, download the sessionTest.php file from the following thread:
showpost.php?p=23826&postcount=2
if this fails, you will need to contact the host to have session management fixed, most likely it is a problem with a bad session.save_path in the php.ini