Reply #20 in that thread has one example of the code I was referring to.
You can send serial data to the Arduino, and have it do nothing but echo the data back.
This allows you to develop the sending application, first. Make the LED On button send . Then, make the LED Off button send . Then, make the slider callback send where the string returned by LedIntensity.Value.ToString();.
When all three callbacks send the right data, then work on the collecting, parsing, and using code on the Arduino, one step at a time. Collect the data, and echo it back to the C# application. Then, parse it, and echo the parsed information. Finally, use the parsed data to control stuff.