that error message means that cURL is not enabled in the PHP.ini settings, it will need to be enabled in the php.ini on the server.
see the following page for details on enabling cURL in xamp.
when editing the server behaviors, are you editing them or adding new ones.
to edit the server behavior, go to Window -> Server Behaviors.
locate the server behavior in the list and double click.
when you double click the server behavior, this should be remembering the existing settings.
it sounds to me like you are clicking the plus button on the server behavior panel to add a new instance of the server behavior rather than to edit the existing instance.