How can you determine the seed to use for level-seed ? And what's the valid level-type for a Biome's a Plenty world?
in game, type /seed and it will give you a string of numbers, that is your seed. Although, if you are uploading a world, then the seed shouldn't really matter that much. Now if you wanted to start fresh, but keep the same spawn/biomes around spawn, then yes, using the seed to generate your map would be useful to have.
For Biome's o'Plenty, it's in the world generation section of your server. Set it to BIOMES_OP (I think.. someone can correct me here if I'm wrong)