Characters play by different rules in Carrara than standard objects. If you want to edit the mesh of the character in it's pose, then you'll have to export the posed character as a .3ds or .obj file and re-import it into the scene. You'll then be able to edit the mesh. 

Why does it work this way? Because the posed characters are an 'interpretation' of the "T" posed mesh that has been modified/morphed by the skeleton.