It is caused by either one of these two things: 1. The name of the folder that the mod is in must be exactly "Caveman2Cosmos", this was a rule before PPIO too as it caused instability to name it something else, PPIO on the other hand demands the name to be correct. 2. If you have the mod in "...\Documents\My Games\Beyond the Sword\MODS\Caveman2Cosmos", even if you name that last folder correctly. The mod must be placed in the folder where beyond the sword is installed on your system. Should be something like: "...\Sid Meier's Civilization IV Beyond the Sword\Beyond the Sword\Mods\Caveman2Cosmos"