Ok, I'll try to describe what I did, maybe it'll work for you as well. Remember, I have no idea what I'm doing, so follow my advice at your own risk.
First, to get rid of the error message we both experienced, I found no other solution than to re-download subsonic and do a fresh install. That should delete all your installed files, including the apparently corrupt DiscogsSearchService.class-file.
After you've re-installed, check that subsonic is once again working. Then, find the DiscogsSearchService.class-file (it's in your subsonic-folder somewhere, a search should locate it). It might be wise to take a copy of this file before you start messing about with it, in case anything gets screwed up.
Open the DiscogsSearchService.class file with a binary editor. I used a program called Frhed, found here:
http://sourceforge.net/projects/frhed/ I guess any binary editor will do. In the program, select Find and Replace, search for the 10 digit API key (I can't remember the number at the moment, but it's written in this thread) and replace it with your own, which you get (at least which I got) from registering at discogs and searching around in there for an API-key. When the search & replace has done it's work, save the file, stop subsonic, start subsonic, and you should be good. At least I was.
Best of luck!