To open an ini file, use either the main menu item File | Open or click on the toolbar [Open...] button, as shown.
A dialog is displayed for navigating to the correct folder and selecting the file to open. As each file is opened, it is added to the "Recent Files" list. The last 10 files opened are kept in the list and any one of them can later be re-opened by clicking on the [Recent Files] button and selecting the file from the list, as shown.
As a default, only files with an extension of ".ini" are displayed in the file open dialog. However, you can choose to display all files by changing the dialog's file type filter.
dcsTools Ini Editor can be used only with ini-style files - that is, files that contain section headings in the format [SectionName] and key/value pairs in the format KeyName=Value. While you may be able to open files of other types, they should not be edited or saved as results are at best unpredictable.