D-Link DIR-640L User Manual page 35

Section 3 - Configuration
STEP.1: If you choose Manually.assign.a.network.key click
next to go to the next step.
STEP.2: Choose your wireless password. You will need this when
connecting to the router from now on.
STEP. 3: Setup is completed, you should take note of your
settings, especially your network name and pre-shared key.
STEP.4: The router must now reboot.
D-Link DIR-640L User Manual 31