W. r. t. to the URL 
https://developer.salesforce.com/docs/atlas.en-us.200.0.fundamentals.meta/fundamentals/adg_relationships_dl_sample_data_try_it_out.htm
I am working with "Data Import Wizard" to import csv files into custom objects "Candidate", "Position" and "Job Application". I followed the isntructions as specified in "Try It Out: Download Sample Data". I have been able to import Candidate and Position records. However it is consistently failing on "Job Application" object. To understand the process, I am attaching the screen shot.

Final outcome is in the result file generated by the wizard and is as follows:
"Id","Success","Created","Error"
"","false","false","INVALID_FIELD:Foreign key external ID: george@schnell.com not found for field First_Name__c in entity Candidate__c:--"
"","false","false","INVALID_FIELD:Foreign key external ID: dcarroll@merce.com not found for field First_Name__c in entity Candidate__c:--"
.....

What is wrong with my process? Please suggest.  Thanks in advance for your kind help in this regards.