Hi Gustaf,
On both my Macs (an iMac and a Macbook Pro, both running OS X 10.6.7) I use the make mac_mpi option without any problem. I have had to modify the Makefile.mac_mpi slightly though, so try changing the following lines to this:
CC = mpic++
CCFLAGS = -O -MMD
LINK = mpic++
meam_SYSPATH = -L/sw/lib/gcc4.4/lib
reax_SYSPATH = -L/sw/lib/gcc4.4/lib
I have a finked FFTW (2.1.5) on my system, and with the above settings I am able to make LAMMPS on my system with all standard packages except the gpu package (I have no use for this). Remember to also make the meam, poems and reax libraries in the lib folder if you plan to use them.
Hope this works for you!
Sincerely,
Christer H. Ersland