The $validators was a cut-and-paste error and was not in my final form code (sorry about that).

Here is the form code that works:

$testing = new Zend_Form_Element_Text('testing');
$testing->setLabel('Test Value')
->setRequired(true);

$testing->addPrefixPath('My','My/');

$testing->addValidator('TestVal');

Adding the prefixPath to the element runs the custom validator perfectly, but adding it to the form as in the previous message it doesn't work.