A couple things to look into:
On our wiki, the account creation page is not accessed as Special:CreateAccount, it is accessed as Special:UserLogin&type=signup. This may be a difference between versions of MediaWiki, I don't know. (I'll note that trying to access Special:CreateAccount on our wiki redirects to Special:UserLogin/signup, so this may not actually be a problem.)
Do you have a line like the following in your LocalSettings.php?
Code:
require_once( "$IP/extensions/ConfirmEdit/QuestyCaptcha.php" );
I think you'll need to load that before you can access the QuestyCaptcha class. Since your directory structure seems to be special, you'll have to modify the above line so it points to the proper location.