Client:
Move the file "GenerateNetherOres_Client.zip" to your "mods"-folder of the .minecraft directory.

Server (Vanilla):
Move the file "GenerateNetherOres_Server.zip" to your "mods"-folder of the server directory.

Server (Bukkit):
Move the file "GenerateNetherOres_Bukkit.zip" to your "mods"-folder of the server directory.