Can't default date to CURRENT_TIMESTAMP in MySQL 5.5
From the MySQL 5.5 manual:


  You cannot set the default for a date column to be the value of a
  function such as NOW() or CURRENT_DATE. The exception is that you can
  specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column.


Therefore, what you want to achieve will work in MySQL 5.5 if you add a TIMESTAMP column instead of a DATE column.

The changes in 5.6.x that allow the functionality are documented here, and I'll quote the relevant summary for completeness:


  As of MySQL 5.6.5, TIMESTAMP and DATETIME columns can be automatically
  initializated and updated to the current date and time (that is, the
  current timestamp). Before 5.6.5, this is true only for TIMESTAMP, and
  for at most one TIMESTAMP column per table.