How do I setup an import for the Do Not Email field?
Welcome to CiviCRM!

For the import of Boolean fields (yes/no, true/false, check boxes, etc.) the correct syntax is 0 for no or false and 1 for yes or true. Use 1 instead of Y or YES, and it should work for you.

Be aware that your imports thus far may have created NULL values in the database. See this question and answer for reference: https://civicrm.stackexchange.com/a/7396/105

You'll want to check your database or have your admin check those values to make sure you've got 1s and 0s instead of NULL.