Since the Incapsula servers are the point of access which is seen by the hosting server, the Incapsula IP ranges appear as the incoming IP in the requests.
If you wish to log the original end-user IP for each request using XenForo, the X_Forwarded_For header should be monitored, as described in these quick steps:
1) Open library/config.php
2) At the end, add:
$_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_FORWARDED_FOR'];