From the left menu, click on "Resources", then select the "Customers" link:
Click on "Create Customer" to add your first record.
In case you already have customers, you will be able to see all customers previously created, as below:
Use the Action Icons to Edit/Delete existing Customers
NOTE Customers already tied to existing accounts CAN NOT be deleted.
Click on the "Create Customer" button to create a new customer. The screen below will be displayed. Fill the fields accordingly:
Click on "Create" to save the changes.