This is a common question and even though the answer is very very simple, for a reason or another, seems that many people get confused on how they can achieve this, so i will illustrate this, step by step with images.
Step 1: Login as a customer into your application and go to your lists:
Step 2: Click on your list title or on the Info icon for your list
(see highlighted section on Step 1)
to see the list overview page from which you will click on the Tools box :
Step 3: Click on the Import box:
Step 4: Choose one of the import options depending on the file you need to upload: