Start by going to https://author.amazon.com/.
Login and you should see your current books listed there. If you have more than one Amazon Author Page, click your author name on top right and load the author page you want to add books into.
Click on the "Books" link at the top of the page.
Click "Add a Book" and put your asin in the box. Click on the book and it will be added to your Amazon Author page.
Keep in mind it could take a few minutes to a few hours for the book to appear on your author page.