I also have a Mac OS X machine, which I recently upgraded to Snow Leopard (Mac OS 10.6). When trying to make the most recent version of LAMMPS I had plenty of complaints about "file is not of required architecture", and also when trying to install other Unix packages.
I am in no way an expert on this, but at least I know it has to do with the 32-bit and 64-bit versions of various packages, and that they do not communicate too well. After struggling a lot with various packages both in general and with the package manager Fink in particular, I did a clean install of my system last weekend, and now there are no more complaints about "file is not of required architecture". It might be that some other fixes or just reinstallation of some packages could to the trick, but I do not have any specific tips, unfortunately. However, I do not think LAMMPS is the problem here - probably there is something strange or wrong in your own system, especially if you have also upgraded to Snow Leopard.
Good luck though!