What Techpriest said. You need:
1) A bukkit port (This is a .zip made specifically for the server. This is what "enables" the mod for online use.)
2) The SP files. (This is the .zip you would install to play the mod in single player EVERY SINGLE PLAYER NEEDS TO HAVE THESE FILES OR YOU RUN THE RISK OF CORRUPTED DATA, NULL POINTER EXCEPTIONS, ETC)