Here is the code of the generator. There are no errors at all

package worldgen;

import java.util.Random;

import worldgen.feature.StructureRuinsmall;
import net.minecraft.world.World;
import net.minecraft.world.biome.BiomeGenBase;
import net.minecraft.world.chunk.IChunkProvider;
import cpw.mods.fml.common.IWorldGenerator;

public class worldgen implements IWorldGenerator {

@Override
public void generate(Random random, int chunkX, int chunkZ, World world,
		IChunkProvider chunkGenerator, IChunkProvider chunkProvider) {
	switch(world.provider.dimensionId){
	case 0:
		generateSurface(world, random, chunkX*16, chunkZ*16);

	}
}

private void generateSurface(World world, Random random, int x, int z) {

	BiomeGenBase biome = world.getWorldChunkManager().getBiomeGenAt(x, z);


	if ((biome == BiomeGenBase.plains)){
		for (int a = 0; a < 1; a++){
			int i = x + random.nextInt(256);
			int k = z + random.nextInt(256);
			int j = world.getHeightValue(i, k);
			new StructureRuinsmall().generate(world, random, i, k, j);
		}
	}



}

}

Did you register it?

Oh my god, I totally forgot about that! Thanks for asking!