Whether you are importing Zip, account, or sales rep data you always need to drag the zip code (fips or post codes) in order for the Next button to become active. As soon as you drag the zip code from the left to the right you will notice that the button becomes an actual button that when clicked will bring you to the import summary screen.
This issue comes about when you have only imported zip code based data and not accounts. This is normal and you have not done anything incorrectly. You simply need to import account data in order to display the accounts on your sales territory map.