Same, I put the difficulty all the way up to Hard and nothing spawned, nothing. I would be happy if just pigs spawn as its a much easier way to get porkchop.
it's because of the way the game works, unless you specifically hack them to spawn something specific, all the monster spawners you place will only spawn pigs On the bright side, if you add some lava/fire around the spawners you can make a cooked pork chops factory
as zelch has said, they are not mob spawners, the only mob spawners you can find are the ones randomly created around the map, the ones you get from the mod tool are pig spawners, also as zelch has said you cannot place down spawners which spawn monsters etc.. although it is very easy to create a "monster spawner" you need alot of light around your pig spawners for them to spawn, i recommend glowstone.