It should just be "q" once the player is running when the developer image is installed. Check to make sure you are using no flow control, (you are seeing the boot messages, so the speed and such is fine). Also, it could be a damaged serial cable. Swap the cable around, and see if the boot messages are no longer displayed, but hitting q once the player boots dumps it into standby. I had a cable with a bad pin, and this was what I observed.

What program are you using? Minicom, or something else?