Hello, I'm having a similar problem. Apologies for hijacking the thread but I couldn't see a button that would let me start a new one.
I get the "unknown error" message too. Until today I had four expansion packs installed and all worked successfully. I then downloaded Late Night off Amazon in Origin Code form. Successfully added the game to my Origin library and then downloaded it. It seemed to install successfully until I went to launch the game and that error game up. After a bit of googling, it looked like the problem might be I installed an 'older' EP after some 'newer' ones. I proceeded to remove everything and start a clean install and install the EPs in the 'correct' order. I downloaded the base game again and this launched /played fine. I then proceeded to install Late Night again (the oldest EP I now had) but the same error happened. I'm a little lost. I also can't find Late Night in my Applications folder, yet could see all my other expansion packs. The only trace of Late Night I can find in Finder is if I right click on the base game --> package contents --> add-ons. How do I get Late Night to work?? Have I got a 'bad' code from Amazon...? Should I have bought direct from Origin?