To search and select a customer, click on the Customer button.  This should always be your first step when entering the point of sale.

You can search by typing into the search field.  Press the enter/return key on your keyboard to run your search and display your customers.  You can use the New Customer button if you need to add a new customer. 

From your search results, select the customer you are processing the transaction for.  This will display the customer card.  Click the plus icon to choose this customer.