I wanted to post this as a (searchable?) reference for anyone out there wanting to output using Serial but not seeing the device connected to your computer.
I wanted to test the USB serial functionality (Serial.begin(), Serial.println(), etc) for debugging code. I had a different sketch running on my Spark when I started to play with Serial. However, if the currently running sketch does not contain Serial.begin(), the Spark will not show up as a COM port/device on your computer (on Mac OS X 10.9 anyways). Once you flash a sketch that contains Serial.begin(), you should see your Spark on your computer after the Spark reboots.
This wasn’t 100% straightforward for me, but it only took about 60 seconds to figure out. Hopefully this tip will help someone else getting started with Serial. It may not hurt to include it with the Serial documentation either.