In order for WHMCS to be notified when a client pays you must set the APC URL in the NoChex control panel. Navigate to Payments Page Setup and enter the URL in the following format in the Call back URL field: http://yourdomain.com/whmcsfolder/modules/gateways/callback/nochex.php
Replace yourdomain.com and whmcsfolder with the location of your WHMCS installation.
The Success & Cancel URLs should be left blank. WHMCS will pass over the desired return urls for the customer after each payment dynamically along with the payment request to NoChex.