Clear Screen
First, you should press CLR so you see the ENTER ID-PIN screen
Enter Pin
Next, enter the eight digit code you were given and press ENT
The first four are your ID and the second four are a PIN that you can change
Remove Keys
Next, you will see this screen. Press the 8/DOWN key until you see CHANGE PIN
Change Pin
To change your pin, press ENT then follow the prompts on the screen.

Some things to remember:
Checking out keys is controlled by user ID, ANYONE can return ANY key
Use your ID only and do not share, you are responsible for the keys you checkout