Inserting a column is not a solution. Problem is the JDO support inserts this column in generated bytecode which works fine in case of DataStore operations. However, with mysql, the column becomes entirely irrelevant.
If you are using eclipse, go to project properties and search for app engine(mind there should be a space between app & engine). Then look for the checkbox which specifies JDO/DataNucleous support. Just uncheck it and rebuild your project. Things should be fine.