First you need to get control of the files view, e.g. like this:

private FilesController GetFilesController()
{
    return SdlTradosStudio.Application.GetController<FilesController>();
}

Then you need to hook into the event, when a file is selected/selection changed:

GetFilesController().SelectedFilesChanged += MyFilesViewPartControl_SelectedFilesChanged;

Then you can do your desired action in your event handler:

private void MyFilesViewPartControl_SelectedFilesChanged(object sender, EventArgs e)
{
    var editController = SdlTradosStudio.Application.GetController<EditorController>();
    // some code to get the selected file/files.
    editController.Open(pFile, EditingMode.Translation);
}

That about what you need?

Regards,
Andreas