As the submission info on their website http://www.mcsweeneys.net/submit/print.html does not specify to whom you should direct your submission, your letter could just be addressed to 'The Editor'. I doubt that Eggers is actively involved in screening incoming submissions.
Good luck! McS's a very prestigious journal, and a real coup to be published in.
LP
Lois Peterson
Writer ~ Editor
Author of 101 - and more - Writing Exercises to Get You Started & Keep You
www.lpwordsolutions.com
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Lois J. Peterson, writer ~ editor ~ instructor
www.lpwordsolutions.com