This question comes up on these boards all the time, but for some reason the search doesn't seem to find it (which might be related to afni.nimh.nih.gov dropping off of google?). It's also not in the FAQs, which is surprising. Try setting your DYLD_FALLBACK_LIBRARY_PATH to the directory where AFNI is installed:
If you are using bash (the default on OS X) and your AFNI binaries are in $HOME/abin:
export DYLD_FALLBACK_LIBRARY_PATH=$HOME/abin
If you are using tcsh (the shell that AFNI likes to use):
setenv DYLD_FALLBACK_LIBRARY_PATH $HOME/abin
Hope this helps!
Isaac