Telnet is a network protocol. You would find setting up a simple serial connection easier.
Enable login on the serial port (ttyS0) by modifying /etc/inittab,
Get a DOS terminal emulation program, like kermit, which is available from
http://www.columbia.edu/kermit/msk314.html
Use a null modem cable, pin 2 connected to pin 3, pin 3 connected to pin 2, and pin 7 straight, for a direct connection, on a 25pin connector. Or buy a straight through cable, and a null modem adapter.
Try baud rates between 9600 and 38400.