This is usually found at: Documents\My Games\Binding of Isaac Repentance\mods .
If you are using a non-Steam version or a mod from a site like The Modding of Isaac : Guide :: How To Install Mods - Steam Community how to install mods the binding of isaac repentance
If the game fails to launch, a mod might be outdated. Go to the mods folder in your Documents and delete the folders manually. This is usually found at: Documents\My Games\Binding of