Here's how to average a set of (previously registered to Tal/MNI
SPACE!) brains and write the results to a minc file that you can then
view in register or Display etc.

In the directory that is one level above the directory where your
_registered_ mri (or pet for that matter) files live, type the
following command: (in this example, the registered files live in
mrital_rsl and the command was executed from
/data/alzheimer/howiedat/nling13/dat)

$> mincaverage mrital_rsl/*.mnc nling13_dat_avg_mri_14s.mnc

I chose this name to reflect that the image was calculated from 14
subjects' info.

For more information, you can look at the help file for the command
mincaverage:

$> mincaverage -help