The skulls can't be accessed without the use of Forge mode. Use Forge on the new Halo 3: ODST maps: Citadel, Heretic, and Longshore.

The skulls resemble those that are in the multiplayer game, "Oddball," and can be used as weapons in a similar way. The difference is that the skulls in Halo 3: ODST do not have an information text box when you pick them up. The 3 skulls are hidden and placed out of the way of the normal paths, which means that you must search for them throughout your maps. Otherwise, you must know where the skulls are. The skulls are like Easter eggs on Easter; you must check every nook and cranny. Watch the video below or for details read the Halo 3 ODST skulls guide here.

Each skull has a different effect on both you and your enemies. So, you must learn how to use each skull wisely. In Halo 3: ODST the skulls are part of the Firefight mode, as a way to increase the variation and difficulty level in this co-op mode.