You need an rs232 shield. You can't directly plug into the serial port due to voltages or some such. I tried something similar, and went through the same pain of not understanding why. Fortunately, the shield is only $12, so it's not a killer. And you don't have to change any code at all.
I would paste a url to the shield, but this forum software won't let copy and paste work in Internet Explorer. I've posted about this before but nothing has been done to fix it. Just search for cutedigi rs232 shield for arduino. Either v1 or v2 will work for you.
Hope this helps.