Hi Emily,
I would recommend you use the omask option, along with specifying which MNI atlas you want to use (-atlas). Aside from the help for whereami, see examples in this
pdf, page 34 in particular.
For the omask option to work, you will need to have the atlas volume on disk in MNI space. AFNI does on the fly space conversions for whereami, but not for the -omask option. So if you don't have atlases on your disk saved in MNI space, let us know which ones you have and which one you want to use and we'll suggest the commands accordingly. To see which atlases you have on disk at the moment, use: apsearch -list_all_afni_P_dsests
cheers,
Ziad