OK, so we know that you CAN run external actions. Now let's get to the heart of the matter:
What is it that you ultimately want this external action to DO?
Also, note that the TEST button is less-than-optimal. In many cases it will not pass the correct variables into the execute action. So you are better off modifying your alert so it triggers for a single node, and only when you click "Clear".
But once again, tell us what it is you want to DO, and we'll start building from there.