I assume that you have all the tools to do this project. You have both .hex files, the one for HID and the one for serial converter. I also assume you have the programming software to switch between the two fuctions so you can easily go back and forth between Serial converter and HID.
After up update the firmware on the serial interface controller, does it appear as an HID device on your computer?
In HID mode, what is the interface baud rate? Are you sure that it is 9600 and not something higher?
You know that you can do
pinMode(switchPin, INPUT_PULLUP);
instead of
pinMode(switchPin, INPUT); // Set the switch pin as input
digitalWrite(switchPin, HIGH); // set pullup resistor
You might want to try something VERY simple like:
Press the button - say "Hello World"
Release the button - say "Goodbye World"
Then work up from there.