It can help us help you if you describe the steps you are taking to create your file.
To create an MPEG in version 12 is to go to Publish & Share/Computer and select the MPEG option. On this screen, you will find a number of MPEG presets, depending on whether you are trying to create a standard definition or high-def file.
What do you plan to do with the MPEG you are creating? That will determine which preset you use.
m2t is mpeg2.