It's easy. You can click on the "Import" tab in the menu and simply paste the addresses in the box.
You can separete the addresses by comma or space.
Than you can press the "Import email addresses" button to send your list to the server.
You will see the list in the "Imported list" box.
After that you can use the action menu and click the "Check emails in the list" option to check your list.
In the "Import" interface, from the upper navigation bar, you can choose the list to rename by clicking the list ID in the "List ID" column, than in the list interface, you can choose "Rename the list" in the Actions menu.
If you checked an email in the past, you can easily use the "Check" option to load from the database the result without use any credit. If you checked the address in the past with the "Check" button, you can force a new check with the "Force check" button, consuming a credit.
You can open the list by clicking the list identification in the "List ID" column, than you can add all your new addresses in the box, then you can press the "Import email addressess" button to add the emails to the list.
You can click the "Check emails in the list" option in the "Actions" button to check only the new email addresses. You will consume credit only for the new addresses.