I did manage to do a javascript error for the How did you hear about us add on as follows:

1. First install How did you hear about us add on

2. Then find

/includes/languages/YOURTEMPLATE/english.php

Search for:
define('ENTRY_REFERENCE_FROM', 'How did you hear about us?');

…right after that line enter the following…

define('ENTRY_REFERENCE_FROM_ERROR', 'You must select a choice from How did you hear about us? pull down menu.');

3. Then find
/includes/modules/pages/create_account/jscript_form_check.php

Search for:

check_select("country", "", "<?php echo ENTRY_COUNTRY_ERROR; ?>");

…right after that line enter the following…

check_select("source", "", "<?php echo ENTRY_REFERENCE_FROM_ERROR; ?>");

That should give a java script error for the dropdown menu itself.

However, I need some help to do a javascript error if someone chooses the "other" option from the how did you hear about us drop down menu but leaves the "other" field blank. Any help would be greatly appreciated.