If your meet has already been seeded and you do not want to reseed to add a new entry, then click Here to find out your Late Entry options. If all of the lanes/positions are full, then you can add an additional heat/flight by clicking the Adjust button in the Run Menu or the Preview or Adjust button in the Seeding Menu. Once in the Adjust Seeding Menu, click on Add Heat/Flight.
Or, from the Run Menu, just press Ctrl-H and MM will add an empty Heat/Flight to the event you have selected. You can also click the + button to the right of the heat buttons as shown below.