The import process only recognizes lookup IDs and not names in order to better handle constituents who don't have unique names.  To import event registrants based on a list of names, do the following:

1.  Create an import process using the Constituent Update Batch template.  The only fields that need to be populated are the Last/Org/Group/Household name and First name with the first and last name of the event registrants.
2.  Import and commit the batch.  Choose the option to create a selection when committing the batch.
3.  Create an export for the new selection and output the constituent name and lookup ID. 
4.  Match up the event registrant import last names and the constituent last names from the export so that the lookup IDs correspond to the correct event registrants.
5.  Use the lookup IDs to identify the constituents in the event registration import.