If updating existing addressees or salutations:
1. Fill the field AddSalImpID with the existing Import ID

OR

If importing new records in addition to updating existing records
1. On the General tab of the Import parameters, select the option to Update existing records 
2. Mark the checkbox to Import records not found as new records

OR

If importing only new additional addressees and salutations:
1. Select to 'Import new records' in the import parameters rather than 'Update existing records'