I haven't changed anything in my project.mml and it correctly references the country boundaries from postgres.
$ cat project.mml | grep "administrative"
"table": "(select way,admin_level\n from planet_osm_roads\n where \"boundary\"='administrative'\n and admin_level in ('0','1','2','3','4')\n ) as admin_01234",
"table": "(select way,admin_level\n from planet_osm_roads\n where \"boundary\"='administrative'\n and admin_level in ('5','6','7','8')\n ) as admin_5678",
"table": "(select way,admin_level\n from planet_osm_roads\n where \"boundary\"='administrative'\n and admin_level in ('9', '10')\n ) as admin_other",
"table": "(select way, name, admin_level from planet_osm_polygon where \"boundary\" = 'administrative' and admin_level in ('0','1','2','3','4','5','6','7','8','9','10')) as admin_text",