I'm running a Tekkit 3.1.2 server, but figured I'd post this in this forum rather than the server operators forum because the issue I was running into was on the client side.
I'm using the alpha SMP port of Mo Creatures 3.7.1 to Bukkit, from http://www.mcportcentral.co.za/index.php?topic=2535.0 . I was able to install the mod to my Tekkit Server and the server booted without errors, loading the mod as far as I can tell.
The issue I'm running into now is installing the Mo Creatures 3.7.1 mod to my Tekkit client, so I can connect to my test server and confirm that Mo Creatures was loaded and is more or less functional. I've tried simply following the instructions from the main Mo Creatures Thread ( http://www.minecraftforum.net/topic/81771-v125-mo-creatures-v371-with-mohorses-donkeys-mules-zebras-zorses-and-extrabiomesxl-support/ ), and I've also tried simply copying over the Mo Creatures/CustomMobSpawner mods from the .techniclauncher/technic/mods to .techniclauncher/tekkit/mods directories, as well as also copying over the technic minecraft.jar and modpack.jar. In the cases where I follow the Mo Creatures main instructions (copying the .zips to the mod directory, and forge/GuiAPI to minecraft.jar), I get a white screen right after trying to launch Tekkit. When I start Tekkit through the Tekkit Launcher if I copied the files from the Technic side, I get to the "Mojang" screen, followed by the error:
42 mods loaded
Optifine OptiFine_1.2.5_HD_S_C6
Minecraft Forge 3.3.8.164
FML v2.2.106.176
Forge Mod Loader version 2.2.106.176 for Minecraft 1.2.5
mod_MinecraftForge : Initialized (modpack.jar)
mod_CodeChickenCore : Initialized (modpack.jar)
mod_NotEnoughItems : Initialized (modpack.jar)
mod_ReiMinimap : Initialized ([1.2.5]ReiMinimap_v3.2_04.zip)
mod_IC2AdvancedMachines : Initialized (AdvancedMachines_4.0_client.zip)
mod_BuildCraftCore : Initialized (buildcraft-client-A-core-2.2.14.zip)
mod_BuildCraftBuilders : Initialized (buildcraft-client-B-builders-2.2.14.zip)
mod_BuildCraftEnergy : Initialized (buildcraft-client-B-energy-2.2.14.zip)
mod_BuildCraftFactory : Initialized (buildcraft-client-B-factory-2.2.14.zip)
mod_BuildCraftTransport : Initialized (buildcraft-client-B-transport-2.2.14.zip)
mod_AdditionalPipes : Initialized (buildcraft-client-DA-additionalpipes-2.1.3.zip)
mod_ccSensors : Initialized (ccSensors-client-MC125-B017pr1.zip)
mod_CCTurtle : Initialized (ComputerCraft)
mod_ComputerCraft : Initialized (ComputerCraft)
mod_mocreatures : Loaded (DrZharks MoCreatures Mod v3.7.1.zip)
mod_EE : Loaded (EE2ClientV1.4.6.5.jar)
mod_EnderStorage : Loaded (EnderStorage-Client 1.1.3.zip)
mod_IC2NuclearControl : Loaded (IC2NuclearControl_client_v1.1.10b.zip)
mod_ImmibisCore : Loaded (immibis-core_49.1.1_for_1.2.5-client.jar)
mod_IC2 : Loaded (industrialcraft-2-client_1.97.jar)
mod_InvTweaks : Loaded (InvTweaks-1.41b-1.2.4.zip)
mod_MAtmos_forModLoader : Loaded (MAtmos_packaged.zip)
mod_ModularForceFieldSystem : Loaded (mffs_rev7_for_1.2.5-client.zip)
mod_IC2_ChargingBench : Loaded (mod_chargingbench-client-1.95b-1.2.5-r1.zip)
mod_CompactSolars : Loaded (mod_compactsolars-client-2.3.2.10.zip)
mod_IronChest : Loaded (mod_ironchests-client-3.8.0.40.zip)
mod_NetherOres : Loaded (NetherOres_Client_1.2.1.zip)
mod_PowerConverters : Loaded (PowerConverters_Client_1.3.4.zip)
mod_Railcraft : Loaded (Railcraft_Client_5.3.3.zip)
mod_RedPowerControl : Loaded (RedPowerControl-2.0pr5b2.zip)
mod_RedPowerCore : Loaded (RedPowerCore-2.0pr5b2.zip)
mod_RedPowerLighting : Loaded (RedPowerLighting-2.0pr5b2.zip)
mod_RedPowerLogic : Loaded (RedPowerLogic-2.0pr5b2.zip)
mod_RedPowerMachine : Loaded (RedPowerMachine-2.0pr5b2.zip)
mod_RedPowerWiring : Loaded (RedPowerWiring-2.0pr5b2.zip)
mod_RedPowerWorld : Loaded (RedPowerWorld-2.0pr5b2.zip)
mod_TubeStuff : Loaded (tubestuff_49.1.2_for_1.2.5-client.jar)
mod_WeaponMod : Loaded (WeaponMod.zip)
mod_WirelessRedstoneAddons : Loaded (WR-CBE Addons-Client 1.2.2.3.zip)
mod_WirelessRedstoneCore : Loaded (WR-CBE Core-Client 1.2.2.3.zip)
mod_WirelessRedstoneRedPower : Loaded (WR-CBE RedPower-Client 1.2.2.1.zip)
mod_ImmiChunkLoaders : Loaded (zdimensional-anchor_rev3.2_for_1.2.5-client.jar)
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT aac9dac2 --------
Generated 9/2/12 12:22 AM
Minecraft: Minecraft 1.2.5
OS: Windows 7 (amd64) version 6.1
Java: 1.7.0_05, Oracle Corporation
VM: Java HotSpot 64-Bit Server VM (mixed mode), Oracle Corporation
LWJGL: 2.4.2
OpenGL: GeForce GTX 470/PCIe/SSE2 version 4.2.0, NVIDIA Corporation
java.lang.NullPointerException
at cpw.mods.fml.common.modloader.ModLoaderModContainer.init(ModLoaderModContainer.java:356)
at cpw.mods.fml.common.Loader.modInit(Loader.java:273)
at cpw.mods.fml.common.Loader.initializeMods(Loader.java:628)
at cpw.mods.fml.client.FMLClientHandler.onLoadComplete(FMLClientHandler.java:223)
at net.minecraft.client.Minecraft.a(Minecraft.java:429)
at net.minecraft.client.Minecraft.run(Minecraft.java:738)
at java.lang.Thread.run(Unknown Source)
--- END ERROR REPORT f7158fc6 ----------
Anyone have any idea what I'm doing wrong, and how to install Mo Creatures 3.7.1 into the Tekkit client?
Thanks!