@mavogs, You can see activity on this port only 1-2 min after beginning reboot. After loading framework, you have ~60sec to enter login and password in console, if not, port will be disabled.
If your USB->Serial adapter, based on FT232 or PL2303 you can use it directly without level shifter. You must connect 1.8V from Kindle to VCCIO pin on converter (For FT232RL it is pin 4).
See my simple schematic: