Most likely you are either not using or an incorrect table prefix.
Open your SMF Settings.php with a text editor, the file should be in the root folder of your SMF install.
It should have something like this:
########## Database Info ##########
$db_type = 'mysql';
$db_server = 'localhost';
$db_name = 'ntwb_smf';
$db_user = 'mydatabaseusername';
$db_passwd = 'mydatabasepassword';
$ssi_db_user = '';
$ssi_db_passwd = '';
$db_prefix = 'smf_';
$db_persist = 0;
$db_error_send = 1;
Use the database prefix shown there in the import settings screen, in my case it is smf_
