The Ore Spiders Mod adds new spiders that correspond with each ore into Minecraft. Their mobs each have a special ability and have a chance to drop their ore. This adds a lot more life to mining and also gives you a chance to collect ores even if you cannot find them.
1. Install the version of Forge that corresponds with the mod (http://files.minecraftforge.net/ Choose the installer version of Forge) Eg. Choose the latest 1.6.4 version of Forge if the mod is in 1.6.4.