First of all, in the JavaScript code you listed in your question, change this part:
onFinish:function () {
window.location = "first_instruction";
}
to this:
onFinish:function () {
window.location = next_location;
}
next_location
will be a JavaScript variable.
Next you need to set this variable within your CakePHP code to value you want. You could do this in the View for each action that uses the flowplayer. The simplest way to put a piece JavaScript into a CakePHP page is to just put something like this into the HTML:
<script type="text/javascript">
var next_location = "the_location_you_want";
</script>