To match your Naviance Unmatched users:
  1. Navigate to Core.
  2. Select Settings > Integration Settings
  3. Click Naviance User Matching.
  4. From the Status drop down, select Naviance Unmatched.
  5. Click Find.
  6. To the right of each user, click Find.
  7. Search for the correct user to match and click Create Link.

If you cannot find a match for a user, it is because this user is in Naviance, but not in your Core database. You'd need to add him first to Core, then you'd be able to match him.