If you have unzipped the mod and you have it in a dir somewhere on your computer and you have no programs open that are currently using it, you should be able to simply copy-paste it to the Custom directory.
If that doesn't work, try copy-pasting it anywhere else on your computer. Make sure you have admin rights to use the computer, or at least such rights that you can save stuff everywhere and not just your "my documents" directory. (even then you can copy-paste the file in the "my documents/my games/ sid's pirates/custom/" directory and it will work...