In most video stores, the id number is the customer phone number, without the dash. So the phone for Linda Wright might be 0000000 in the customer id. Or, you may decide you want to have a serialized number assigned to each of your customers when you first add them to the file.
This lookup actually looks at their Home Phone number. You must type in the dash, 000-0000. Once you've found the customer, press enter, double click, or click on select and that customer will be selected, and the customers actual ID number will be displayed, and you will be taken to the first POS Screen.