Sound pack?
I have not heard anything about that :S
But if you thinking of adding music to LFS you have to convert an music file (MP3 etc etc) to .ogg files (Ogg Vorbis). This can easly be done with a music converter program.
When you have converted a song you put into the OGG folder. (Your LFS main folder / Data / Ogg).
Put it in data/eng, and then in-game type Shift-A, and make sure you're in the right car, and at the top right you should see the file chosen, just click on it and there'll be a list of downloaded engine sounds.
Can't think of any reason why it would say that unless you're trying to run it out of the zip file, but now it unzips itself so I have no clue, is it not running at all?