If you are trying to migrate MassMailer data from one instance to the other instance of Salesforce, please migrate all the custom objects packaged with MassMailer. 

You can find the same by going to Salesforce Setup -> Installed Packages -> MassMailer -> View Components -> Ctrl-F 'Custom Object' and you will find all the custom objects.

You can use any data migration tool such as Salesforce Data Loader to migrate the data.