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.
Non internet accessible machine
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.
URLs not setup correctly
Many components in the system, including our module, make use of the System URL to generate URLs required.
Navigate to the General tab in the WHMCS admin console and ensure that you have set up all your URLs correctly.