Bit late to reply now, but let me answer your question in case anyone else runs into the same problem. There's no executable indeed, as this is an interactive DVD. You can play the game with VLC Media Player:
Extract the contents of the ISO with a file archiver like WinRar or 7-Zip
Start VLC Media Player
In the top bar of VLC, navigate to Media -> Open folder
Select the VIDEO_TS folder of Hourglass of Summer
If VLC asks you to select a file, select VIDEO_TS.IFO
The game should start now. The menu is rather unwieldy, some options being invisible until you hover over them. To play with English subtitles, in the main menu select Configure -> English Subtitle.
You can't save the game actually. DVD-based VNs and other instances where it was expensive/impossible to save data used passwords, i.e. a code that would let you return to a specific point in the game. You can get there from the menu, it's between 'Start' and 'Configure.' It should look something like this:
The menu is rather unwieldy, some options being invisible until you hover over them. To play with English subtitles, in the main menu select Configure -> English Subtitle.