With the Twitter integration feature in WHMCS, you will need to input your Twitter username into the given area.
Please go to the Setup tab in Whmcs and click on General Settings. Once in the general settings area, please click on the Social tab and simply type in your username where it is asked for. Save the changes and that is that.
From there, you will need to be using the new Default theme which was release in Whmcs V5.0. If you paid for a WHMCS theme integration into your website, I am sure you can ask to have them place that module into your current setup.
Also, after looking at your website, it looks like you will need to have the module added to your integration.