Plugin Captcha (Bestofworld)

I have installed Captcha plugin and want to use it for a custom contactform which I made in PhP. http://www.dakterras.nl/offerte/
You´ve given some code to embed this captcha in whatever custom made form.

The first line stands in the php-file of my custom form now.
if( function_exists( 'cptch_display_captcha_custom' ) ) { echo "<input type='hidden' name='cntctfrm_contact_action' value='true' />"; echo cptch_display_captcha_custom() } ;

But where do I put the second line ?

You talk about it in FAQ on wordpress.org in this phrase, a bit vaguely:
…Then you should add the lines to the function of the entered data checking…
if( function_exists( 'cptch_check_custom_form' ) && cptch_check_custom_form() !== true ) echo "Please complete the CAPTCHA."

Can you help me out here ?