Suma group analysis: how to do group analysis on surface

Dear professor:
I want to do group analysis on surface.
I create standard 60 mesh surfaces of each individual subjects for group analysis. And I use 3dVol2Surf to map the stats.${subj}_REML+orig onto cortical surface. Then I try to use 3dttest++ do do group analysis with each subject’s surface data. But when I use the option 3dttest++ -Clustsim and 3dttest++ -ETAC. It dosn’t work and give error: ** FATAL ERROR: You cannot use ‘-Clustsim’ option except on 3D datasets /:(.

So I want to know how to do group analysis with surface data and get the Cluster threshold file.
Thanks very much!