When you see a notification, swipe your notification bar down, and tap it to open the Infinit.
In the home view (first tab), you will notice a new card at the top of the screen. The card will appear open, displaying thumbnails of the files included in the transfer as well as a a button to accept and another to decline. Tap Accept to receive the files.
Once the transfer is over, you can access the files in the files view (second tab) or by tapping the card, and then tapping a file.
If you missed the notification, you can always open the app and Accept or Decline the files directly from the main view.