Decessei

When you have the mod, make sure it's the version for Minecraft 1.2.5, v7.1. It's just a bundle of class files, so you should have no trouble running it by placing the .zip in your "mods" folder (.technic/tekkit/mods) The Minecraft Forum thread for the mod can be found at: http://www.minecraftforum.net/topic/964897-152-spmplan-clay-soldiers-mod-v-911/

Looks like you have an ID conflict. This has happened to me before, resetting the modpack usually does the job. If that doesn't work, try backing up the config files for the pack (located at your %appdata%/.technic/tekkit/config directory), deleting them and running the mod again. If THAT doesn't work, just redownload it.