If you have a supported RTMP server http://www.videowhisper.com/?p=Requirements
, first make sure it's running and accessible.
Also, make sure you install rtmp side as explained on:
http://www.videowhisper.com/?p=RTMP+Applications
And fill rtmp address as explained there rtmp://ip-or-domain/videowhisper .
As forums are not monitored, for assistance you can contact support team from http://www.videowhisper.com/tickets_submit.php
Latest plugin version fixes issues in latest Oxwall updates.
For support, contact directly from VideoWhisper site as forums are not monitored.