From Repair Mobile

How to Enter Unlock Codes on Treo 750: (alternative instructions):

1. Please insert a not allowed SIM card

1. Place the battery and the cover, windows will boot automatically

1. Turn on the phone by pressing the front upper-right red button on the keypad

1. The phone will ask for the unlock code

1. Pull out the stylus at the back of the phone, tap the keyboard icon at the center-bottom of the screen More details [1] - thanks to cellphonerepair.org