How to download Mo' Creatures:

DONT PUT ANYTHING IN MINECRAFT.JAR YET!

First download Minecraft Forge:
Minecraft Forge (Adfly Download) (<- Ad link removed)
Minecraft Forge (Direct Download)
Minecraft Forge (Website)

Then Downlaod GUI Api:
GUI Api Download Page

After that download CustomMobSpwaner 1.5.1:
CustomMobSpawner 1.5.1 Download

Then Download Mo' Creatures:
Mo' Creatures v3.7.1

1. Extract Your Minecraft.jar

2. Put Forge in your Minecraft.jar

3. Put GUI Api in your Minecraft.jar

4. Put Mo' Creatures into the mods folder in .minecraft or if your on a mac, Library/Application Support/Minecraft/mods

5. Put Custom Mob Spawner 1.5.1 into your Minecraft.jar

6. Run your Minecraft.jar

7. Play with turtles!

(If you have any problems put them in the comments section and I will respond and help!)