Twilio Extension - key objects are empty

[SarahG (FountainTribe)]

I an attempting to send SMS messages using the Twilio extension ( version 1.0 ) and CiviCRM version 4.3.8.  I have configured it per the directions on the civicrm wiki (http://wiki.civicrm.org/confluence/display/CRMDOC/Setting+up+a+SMS+Provider+for+CiviSMS)

But it does not work. I have added debugging code to the "send" method of the extension in the file "org_civicrm_sms_twilio.php" , which is showing that the main objects needed are empty/null. ( ie the variable at:  $this->_twilioClient   )

I initially configured the SMS provider to use the API type "XML" per the wiki instructions.  Out of lack of ideas, I tried using the API type "HTTP", but that did not make any difference in my results.

What can I try next? Any ideas on how to troubleshoot this further?