I am having trouble getting AMIDST to load a level.dat file.
I finally remembered that it only seems to work on a vanilla minecraft.jar, but it still wouldn't load the level.dat file.
So then I goggled how to find a world's seed, but I only found some online tools that also didn't work.
All you have to do is click Re-Create your world in Minecraft, then you can look at the seed and options under More World Options. Then just cancel and don't recreate the world.