Not able to migrate attachments

I am attempting to migrate attachments. I have imported an initial batch of test leads, which went fine. Then went to migrate the attachments for those leads.

I am using the Lead ID from my export report as the Parent ID for my attachments... i.e.

zcrm_2011028000000104001

I use this ID as the parent ID in the csv file, but I get this error message. "Could not find a matching parent record for this look up field - Parent ID"

Please help.