can you check your wowBB tables with phpMyAdmin? the table wowbb_attachments should have a column "attachment_id".. also the wowbb_posts. is there anything related in the posts table?
If you don't use attachments in wowBB you can also remove the complete attachments block from our converter:
/******************************************************************************/
--- Converting attachments ...
/******************************************************************************/
---* {$to_prefix}attachments
---{
$no_add = true;
$keys = array('ID_ATTACH', 'size', 'filename', 'ID_MSG', 'downloads');
$newfilename = getAttachmentFilename(basename($row['filename']), $ID_ATTACH);
$file=fopen($attachmentUploadDir . '/' . $newfilename,'wb');
fwrite($file,$row['file_contents']);
fclose($file);
@touch($attachmentUploadDir . '/' . $newfilename, filemtime($row['filename']));
$rows[] = "$ID_ATTACH, " . filesize($attachmentUploadDir . '/' . $newfilename) . ", '" . addslashes(basename($row['filename'])) . "', $row[ID_MSG], $row[downloads]";
$ID_ATTACH++;
---}
SELECT
a.attachment_id, a.file_contents, p.post_id AS ID_MSG, a.file_name AS filename,
'' AS size, a.downloads AS downloads
FROM {$from_prefix}attachments AS a
INNER JOIN {$from_prefix}posts AS p ON (a.attachment_id = p.attachment_id);
---*