Yep, that looks like it's my problem. Sorry for the crap bundling.
It works on mine because I have that library in that location. The bundle does contain that library, but the executable must be looking for it in the other location. A quick test would be to copy the libSDL-1.2.0.dylib (in FS-UAE/Contents/Frameworks/) to /opt/local/lib/.
When I have time, I'll do it properly.