Although not traditional IF in a sense that there is no text parser, President is Missing and its sequel Presumed Guilty embody what make great IF: involving story and inventive puzzles where logical deduction and attention to detail are key to success.
Both games cast you as official investigator: President is Missing puts you in charge of an investigation to rescue missing presidents from terrorists, while its successor is based on a more typical detective plot. Innovative on many levels, including real-time action, great details, and captivating storylines. Check out the "database" command for comprehensive on-line instructions and background story.
Note: In addition to the manual below, you also need the cassette tape that came with the game, since you must decipher the morse code at one point. You can download the cassette content in mp3 format in the "Extras" section below, and listen to it with any media player.
Review By HOTUD
Got the cassettes rip from this blog, which also has a quick review of the game.
English 
Comments and reviews
I can't find any mention of this game on the site where you say it's being sold. Probably it got removed because the developers intentionally never programmed an ending. Maybe it's safe to put it back on.
Thanks @moonbug, I uploaded the mp3 here as a mirror.
The audio tracks can be found here:
http://trixter.oldskool.org/2010/07/31/stone-age-multimedia-the-president-is-missing/
That still leaves one problem: there's no real game ending, but rather you're supposed to mail your conclusions to the publisher (who no longer exists). With some luck, maybe there exists a document somewhere on the Internet that you can check and compare your answers with...
the president is missing?
missing what?
everything...
No cassette tape content to download
No comment nor review for this game at the moment.