Do you have a userid field defined in your .cfg file?
It would help to see your .cfg file to see how your fields and permissions are setup. Could you save a .txt copy and post the url to where it can be viewed.
Be sure you also have set in your .cfg file:
$auth_user_field = 0;
changing the 0 to match the field number of your userid field. I have this working in several databases with no problems.
Remember this will only work if the user is required to login to add records.
Unoffical DBMan FAQ
http://redundantcartridge.com/dbman/