Looks to me like in your edits you deleted or missed adding a <?php opening tag. I don't know anything about HTC, but my guess is that at some point in the instructions, you needed to add the code above - locate the relevant file, and find the code. Just before it, try adding an opening tag so that it reads:
<?php
if (isset($HTTP_GET_VARS['manufacturers_id'])) $db_query = tep_db_query("select manufacturers_htc_title_tag as htc_title, manufacturers_htc_description as htc_description from " . TABLE_MANUFACTURERS_INFO . " where languages_id = '" . (int)$languages_id . "' and manufacturers_id = '" . (int)$HTTP_GET_VARS['manufacturers_id'] . "'"); else $db_query = tep_db_query("select categories_htc_title_tag as htc_title, categories_htc_description as htc_description from " . TABLE_CATEGORIES_DESCRIPTION . " where categories_id = '" . (int)$current_category_id . "' and language_id = '" . (int)$languages_id . "'"); $htc = tep_db_fetch_array($db_query); ?>
Be sure to back up the files first, so you can revert if it all goes screwy.
Cheers,
Max