This is due to one of two reasons: you are testing from a non internet accessible machine (eg. a local development machine) or you have not setup your URLs in WHMCS correctly.
The module will only perform as expected if you are testing from an internet accessible machine (eg. http://www.mydomain.co.za). It will not work from something like http://localhost or http://my.machine.local.
Ensure that you are testing on an internet accessible machine.
Navigate to the General tab in the WHMCS admin console and ensure that you have set up all your URLs correctly.