I think PATH should only contain folders, not the full path to your binary. But now I'm not sure if even that would help. It would be the best if you could join #lispgames or #lisp on freenode so we can debug it in realtime. We might even find someone there who has more experience with cffi.
Static-vectors, on which Sketch depends, needs to have a working C toolchain (C compiler, linker, etc.) installed. Maybe that's the problem here?
If you do have the toolchain installed, and can call the compiler with "cc", then maybe sbcl's PATH is wrong. You can try copying your env PATH to .sbclrc:
Hi all, author here. Thanks! Finding Sketch on top of HN was very unexpected. If you need help running it, have suggestions, or just want to chat, ping me at #lispgames on freenode, or here.