login - Tried to reset Admin PW, but Ubuntu isn't recognizing Admin account. How do I fix this?

I recently got a setup with Ubuntu v15.04, but I don't know the admin password. I have some background in IT, and know how to reset it. (BIOS, hold Shift, Adv. Ubuntu -> Memory -> Root, type passwd *root* where root is account name, enter and verify new password.)
However, when I enter passwd Aviator (account name), it says 'Aviator' is not a user. I need this computer, how do I get Ubuntu to recognize the account?
Thanks.
NOTE: The computer does not have WiFi or Ethernet. I CAN access the Guest account and use Terminal, but it has been no help. I'd really rather not have to hack my own computer...

EDIT: Found true username of admin, 'fltsim01'. Entered passwd fltsim01, was asked for New Unix Password, then to Verify, got the following:

passwd: Authentication token manipulation error
passwd: password unchanged
root@fltsim01-MS-7593:~#

(Note: root@fltsim01-MS-7593:~# is given at Terminal startup, and whenever ready for new command.)

Eyoung, do you want the whole thing? It's 24 two-word lines.