You don't need Norton Commander to run DOSBox. You can still run it just fine without NC once you have the config file set up. However, since the links on this site to NC aren't working, you can find NC (English)
here if you want to use it. It's a slow download, so be patient.
As for your file issue, right click on the file, select "Properties", and make sure it's not set to "Read Only". If it is, uncheck that box and click "Apply". Things should work fine then. If on the other hand, you can't save because an unknown program is using it, just do a quick restart of your PC, and the text file should be freed up.