MarcEdit allows users to select particular records in a MARC file to edit. Whether you have already opened the file in MarcEditor or not, the steps are very similar but in the interest of brevity they are combined in one step.
Select the yellow folder next to Source MARC File at the bottom of the screen. Navigate to and select the MARC file you would like to edit. Once you have selected it, the directory to that file will appear. If you have opened this from the menu of an .mrk file, then there is no need to specify the source file as the filepath for the file you have opened will be already in the text field as it should be pre-populated with the correct filepath.
Notice that the numbering of the records starts with zero.