All records in Salesforce are identified internally with 10 digit alphanumeric system generated Ids.  When updating Salesforce records via data loader when need to give data loader that 18 digit Id for it to know what record to update.  Additionally, if you are trying to update a look up field, even though that look up field on the salesforce screen shows a persons name, behind the scenes there is actually an ID in that field.  

If the hiring manager info you are trying to update is a Salesforce user, the ID you need is a User Id.   If the hiring manager is a contact, you need the contact id. 

When trying to import data via data loader, inorder to get the salesforce Id associated with source data, many people usually do an export out of Salesforce (you can use data loader to do that) of the ID of the Salesforce object they want plus some additional field to use to match up against external data inorder to join the SF id to the data.   For example, if you have a excel spreadsheet with contacts to import, and the contacts email address is in the spreadsheet, export contact's Id and email address out of salesforce and then use excel lookup functionality to match the source data's email address to the sf exproted data and associate the sf Id to the source data in the excel spreadsheet.

Also, there are other tools that may allow you to do this easier specially if you are not familiar with data loader or salesforce.  Data loader while it works well, is not the most intuitive tool.  Look on the Salesforce appexchange for these tools.  Also, Salesforce used to have data import tools within the app, but honestly I am not sure if they are still there, I never used them.

Hope that helps.