There is only one time and one way to pilot the Scarab in Halo 3. On the Ark, when you fight the Scarab, this is how you do it: Kill everyone, everyone including Covenant and UNSC. Everyone, alien and human, on the field. Then get on top of it, walk to the front (above the "eye") and any direction you turn, the Scarab will too.
This is because the Scarab tank is ment to follow you, if there aren't any other people on the field including humans and aliens. Then the Scarab will "follow" you. So technically your piloting it.
Or, if you want complete freedom in piloting it, the only way is to download Halo: Custom Edition(its only for the PC) and go download some maps, preferably the one named "Extinction" at halomaps.org.