Are these the instructions you are following? Othere than #1 since you don't have a Cingular Sim, maybe you can borrow one from a friend to unlock it or try another SIM, but you only have 5 attempts at entering the unlock code.
I got mine unlocked today, free, from Cingular, and he even stayed on the phone while I unlocked it to make sure it was good
PS. The instructions excluded item #3a
1 Insert Cingular SIM into device
2 Power phone on and Turn off the radio (Turn Wireless Off option) VERY IMPORTANT.
3 Select xxx8220;Optionsxxx8221;
3a Select "Advanced Options"
4 Scroll down and select xxx8220;SIM Cardxxx8221;
5 Type xxx8220;MEPDxxx8221;
6 Type xxx8220;MEP2xxx8221;
7 Enter the Unlock Code
8 Press enter
9 Reboot device. Device is now unlocked.