First make sure your CSV file is formatted with the first column as First Name, the second column as Last Name, and the third column as Email Address.
Then you navigate to "Senders" and press the "Add new Senders" button.
On this page it will offer you the option to manually add a sender or load a CSV.
Then under the “Upload Senders” heading there will be a “Choose File” button.
Note: In order to be sent the email message must have a plain text element to the content, straight HTML will not be recognised.