1. Hit Segments on the left-hand side

2. Then click New Segment 

3. From here, name the segment and then hit create and detail:

4. From here, scroll down to the bottom of the page and hover over tasks. Then choose import list:

5. Then upload your file:

6. From here, certify your emails are good for import and you'll now have a group of contacts that you can select to send an email to.