Well they are right you only need one vector to determine the sun direction. You want the inverse of this vector to point the light in the right direction. So align -lx,-ly,-lz. I am not sure what the implications of +Z being up are at the moment. Maybe you align to -lx,-lz,-ly instead of -lx,-ly,-lz.
hah. Stevie beat me to it again, and his reply makes more sense to boot.
hah. Stevie beat me to it again, and his reply makes more sense to boot.