I also submitted a support ticket on Sunday 12th for another problem and it remains 'open' and not assigned.
I'm really sorry about that. I have school during the week, and free time is fairly limited. I'll look at it tommorow, about this time.
Make sure you're typing the cPanel command correctly. It should look like:
/usr/bin/php -q /home/username/public_html/forum/sources/Shop/dointerest.php > /home/username/public_html/forum/interest_log.htm
username is your cPanel username, and
forum is the directory you placed your forum in. Set this to run every day at midnight. If you're still having trouble, I'll make some screenshots for you.
--daniel15