There currently seems to be a bug in Minecraft that may adjust the biome type of your world after you import your preset into the game. The good news is that it’s predictable, and the better news is that there is a workaround. If you select a biome of FrozenOcean or beyond (fixedBiome of 8 or greater), Minecraft will add 2 to fixedBiome once you click “Use Preset” in the game. Most of the time this will mean Minecraft will want to use a different biome than you intended. However, if you selected either of the last two biomes (Mesa Plateau F or Mesa Plateau), the game will crash.
Fortunately the workaround is simple. Once you click “Use Preset,” adjust the biome slider in-game to get the biome you wanted; it will usually be two ticks to the left. Just remember either Mesa Plateau will crash the game, so don’t try importing those. If you are using any biome before FrozenOcean, including “None,” then no adjustment will be needed as the bug will not be triggered. If you plan on saving your preset, please do not make any adjustments on the site to avoid sharing presets that deviate from their intention.
At the time of this writing, the current Minecraft version with this bug is snapshot 14w21b. It’s important to understand that snapshots are really pre-release versions, and there stands a good chance that this bug will be corrected when 1.8 is released (There is already a report on Mojang’s bug tracker page). I’ll update this post with any developments; sorry for the inconvenience.