Double clicking on the file should automatically open an editor such as Notepad or WordPad.
Type away, cut,copy&paste and then save.
NotePad and Wordpad come with windows, so you should have them.
If that fails, hold down the shift key, and right-click on the text file.
In the menu that pops up, choose "Open With..."
Then scroll down through the options and select Wordpad. Check the "Always use this program" box, and hit OK.
From now on, double clicking on any text file will bring up the editor.
When editing SE4 files, it is best to copy pre-existing entries, and modify them, to save on typing and ensure you get everything you need.
Be sure not to leave two blank lines in a row, since SE4 will choke, and ignore everything after that point.
Also, see
this thread for info on how to create a modfolder, so you don't have to risk losing your original SE4 files.