You can do one of two things. Either align your data to a Talairach template using @auto_tlrc and then transform the data to MNI space using 3dWarp.
@auto_tlrc -base TT_avg152T1+tlrc. -input anat+orig. -suffix _tt
@auto_tlrc -input mask+orig. -apar anat_tt+tlrc -suffix _tt -dxyz 3
3dWarp -tta2mni -NN -prefix mask_mni mask_tt+tlrc
Alternatively, align to a native MNI template included with FSL or SPM or transform a Talairach template provided with AFNI.
@auto_tlrc -base avg152T1+tlrc. -input anat+orig. -suffix _mni
@auto_tlrc -input mask+orig. -apar anat_mni+tlrc -suffix _mni -dxyz 3
See the help for @auto_tlrc for more information.