There's a folder-shortcut "AppData" in your C:\Program Files\ShippingExplorer-folder which will take you directly to your user folder with the myfleet-file.
If you can't find the shortcut, press Win+R on your keyboard and type in:
and press Enter.
но его редактирование предусмотренными методами довольно затруднительно, с файлом было работать намного удобнее.
What is the purpose of editing the myfleet-file yourself? Editing the file is not something you should normally need to do and is not meant to be edited yourself, instead let us know what it is you can't do in the application directly and we would try to add the functionality in the application that you're missing.