These directions will only be helpful if your PC has an old-school serial port, and even then it's a lot of work. It's far easier to purchase a USB to +3.3V level shifter to get the job done.

To adapt 3.3 TTL logic from NAS to +15/-15 PC serial, you can build your own converter. This schema is from Sodoityourself with two errors corrected and explicit pinout for DB-9 female.

Please note that MAX232 circuit can't use 3.3 Vcc from serial Vcc, you must pick up 5V elsewhere (on board or from external power supply), or else use a MAX3223, that accepts 3.3V power supply. The original schema came with Rx and Tx swapped one with each other, have a look to your level converter's datasheet if any doubt.

This schema is easy to apply on a evaluation board.