Do you see a pink button My account in the top right corner of your screen? That means you are already signed in. Do you see the button Sign in? Then you have to log in first.
When signed in, click the pink button My account in the top right corner of your screen.
Click the black button Change/Add under the header Addresses.
Click the black button Add address.
Submit your details. Click the black button Confirm. You have now successfully added an address.