Hi Suzy,
I remember a posting on the message board about using the AFNI dataset TTatlas+tlrc to create masks of Brodmann areas. I recall Bob Cox saying he didn't advise doing it, but I couldn't find that post. I can tell you how to do it, but with that caveat. Perhaps Zorkon would like to weigh in on this again for those of us with short memories.
AFNI FAQ 80 has a link to a document that describes the TTatlas+tlrc dataset, but when I clicked on it, I got a 404 not found error. The dataset has two sub-briks, each defining regions of the brain. Sub-brik 0 gives separate values to large regions, such as middle frontal gyrus. Sub-brik 1 gives separate values to each Brodmann area and to other smaller structures, such as thalamic nuclei. You can use 3dcalc to make a mask of one of these regions as follows:

3dcalc \
-a1 TTatlas+tlrc \
-expr "step(147-a)*step(a-145)" \
-prefix nucleus.accumbens.mask

This would make a mask for the nucleus accumbens, which has a value of 146 in sub-brik 1 of TTatlas+tlrc. (N.B. The TTatlas+tlrc dataset uses the small tlrc box. You may need to use 3dZeropad to add slices to the inferior edge in order to make it compatible with other afni datasets)
Sally Durgerian