The title basically says it all. What I have so far is a class extending biomegenbase, and instance of it, which I add to the biome dictionary and do

ModBiomes.testBiome = new BiomeGenPenIsland(Integers.testBiomeId);
        BiomeDictionary.registerBiomeType(ModBiomes.testBiome, BiomeDictionary.Type.MUSHROOM,
                BiomeDictionary.Type.MAGICAL, BiomeDictionary.Type.SWAMP);
BiomeManager.addSpawnBiome(ModBiomes.testBiome);
        BiomeManager.addStrongholdBiome(ModBiomes.testBiome);
        BiomeManager.addVillageBiome(ModBiomes.testBiome, true);

. But I can't find the biome anywhere after literal hours of exploring, in which I found 2 mesas and 5 roofed forests, which are supposed to be rare. If you have a working biome added to the main world, please tell me how you have done it. Thanks!

http://www.minecraftforge.net/forum/index.php/topic,17885.msg90964.html#msg90964

Thanks a lot diesieben07. I added it to all the lists just to see what would happen, and the results are quite cool.