The issue is that CAT12 generates the mesh in GIfTI format (e.g. "lh.central.cr.gii"),but the corresponding thickness file (e.g. "lh.thickness.cr") is saved in FreeSurfer CURV format, but does not attach the ".curv" extension. The next release of Surfice will be better at automatically detecting these mis-named files, but for the moment you can append the ".curv" extension to your thickness file. You can then open the GIfTI file you want (e.g. "lh.central.cr.gii", "lh.sphere.cr.gii") and then use Overlays/AddOverlays to add the thickness file. You will probably want to change the color map (e.g. to ACTC) and the range (e.g. to 0..6mm).