L Led wont stop flashing.

I just got my Arduino board, so I'm pretty new to this. The yellow/orange "L"LED won't stop blinking.I'm trying to set up a heart rate sensor, but for now, I'm just using someone else's code. Sometimes it says Done Uploading, but keeps blinking, but most of the time it says something about a problem uploading to board. Also, even though this is arduino, whenever I run the processing code, it says - Error, disabling serialEvent() for /dev/cu/usbmodem621 null. Please Help me!

Sounds like you are doing something wrong. Unfortunately you left out any details that might help pinpoint your problem. What model Arduino? What version of the IDE? What operating system? What sketch are you trying to upload? What error do you get when it doesn't work?

Until you get the Arduino sketch uploaded there isn't much use in running a Processing sketch that tries to talk to the Arduino sketch.