From My Lists click on the title of the list you wish to import leads to
​
​

Click 'Subscribers' and from the drop down list choose 'Import'
​

Click on the location box and when the window opens choose the file containing the leads and email addresses you wish to upload to your list and click 'Open'
​
​

Your file should have 3 columns, Email, First Name and Last Name so it will look like this.
​
​

Here you can see the file name you uploaded and when your ready click 'Import'
​

You can then see the progress of your upload
​

When upload in complete you will see this screen and just click 'OK' to close
​
​