Go to Lists and click Manage.
Locate your list and click in the Actions column. This will take you to the list's property page.
Click Add subscribers in the Next available actions section at the bottom of the page. This will take you to the next step where you will have 2 options to upload your subscribers.
Option A) Upload a CSV file
Browse and select the CSV file you wish to upload.
Once uploaded, match the source (data columns from your CSV file) with the corresponding destination (list fields created in Step 2) and select the publication you wish your contact list to subscribe to (the default publication is Newsletter).
Option B) Insert manually
Manually enter your data in each field.
For both import options, as well as for automated imports, you can immediately create a brand new publication by clicking Create in the publication drop-down list.
Once your list has been properly configured, click I agree to confirm that your contacts have expressed consent opting in, then select Save and Continue to import your data.
Once imported, you will be directed to the import results page.