it is in the header.php of your theme.
For modern theme in the header.php /oc-content/themes/modern/header.php :
Go line 41 replace <a href="<?php echo osc_register_account_url(); ?>"><?php _e('Register for a free account', 'modern'); ?></a>
With<a href="<?php echo osc_register_account_url(); ?>"><?php _e('Post an item', 'modern'); ?></a>
If your problem is solved mark it solved please...
For the location what do you want to do: remove it ?
Regards,
Van.