How do I undo the last Add-Migration command?

If you haven't used Update-Database you can just delete the migration file. If you've run the update you should roll it back using Update-Database -TargetMigration "NameOfPreviousMigration" then delete the migration file.

Reference:

http://elegantcode.com/2012/04/12/entity-framework-migrations-tips/

If you haven't executed the migration yet with Update-Database, you can run Add-Migration again with the same name (you may need to use -Force) to re-execute the scaffolding. This is noted in the output of the Add-Migration command.