Once enabled, a WhatsApp icon should appear in your share links on your phone/tablet. It is hidden by default on desktops because clicking the icon on desktop does nothing and the ability to share has not yet been added to WhatsApp Web. This is toggable in the plugin settings.
Use the 'whatsapp_send_text' language string to customize the text sent to WhatsApp. If need be you can customize the template to change order of title and URL.
After clicking the link it opens WhatsApp and you will need to click on a user/conversation for the text to appear.
As of 1.0.4 you must delete the /your-root-dir/system/Content/ShareServices/Whatsappcvh.php file if you've upgraded from a previous version. This may require re-enabling WhatsApp in System > Promotion > Sharing.