Are these files you've created yourself, or are these files you've downloaded from other sources?
Sometimes, especially with older files, there are addons/plugins that you have to have installed in order to open files. Some VERY old ones that you actually have to use an earlier version of Construct for.
Have you tried creating a file, saving it, and opening it? Are you able to open them then, or will it still not work?
In any case, I think you should post a screenshot of what happens when you try to open a file.