If you want to rename a single file then don't just the MultiRename tool. It is used when you want to rename Many files based on some rules.
For single file rename press Menu>Tools>Rename current file. or Shift+F6, Or slow Click and wait on an item.
For the Mass renamer.,Then [N] tag does not need to be there. It just represent the current file name. you can type "MyFile" instead but if you got multiple files in the list then you will get a conflict since not all the files can be named the same then you need to add a [C] (counter) tag so the files get a number too or something.