modify 30W LED driver for variable current output
I don't think the schematic from the application note is intended for high power LED driving. Usually for LED driving, this is done with a power converter configured as a constant current source. 

Depending on your experience with power electronics, I'd caution you moving forward with this circuit. There are a couple things you'll need to think through before you even do this:


You'll want an isolated power source of some sort so you're not directly connected to mains for safety purposes and measurement (unless you have differential probes)
Do you need the isolation that the flyback provides? There are boost converters that are specifically made for constant current driving LEDs with a CTRL pin for dimming. You could have your microcontroller interface with a DAC and drive this pin.
If you do need isolation, you could always take the output of your circuit and connect it to the input of an LED driver designed for this type of circuit.


Example LED Driver (Boost Converter)



Here is the LED current vs. CTRL voltage I was talking about.

If you are set on using this circuit, then you can read this paper on implementing a constant current source:

https://www.fairchildsemi.com/technical-articles/Low-Cost-Isolated-Current-Source-for-LED-Strings.pdf

I think you'll realize quickly that it's not going to be as simple as you thought.