Go to https://author.amazon.com/.
Click "Join" on the top and you will be directed to logging in. Use your normal Amazon account login information your use for KDP and Amazon.
Once logged in you click "Books" and enter your ASIN into the field to add them. You can easily get your ASIN's from your KDP dashboard under your bookshelf. Keep adding your books until they all have been added.
They can take a bit of time to appear, so be patient. Not everything is instant.