add icon back onto dock

Go to your Applications folder, then to the Utilities folder inside that, and find System Preferences. Drag it back to where it was on the Dock. Then, just in case you're tempted to remove it from the Utilities folder, since it's "safely stored in the Dock" - DON'T! The Dock is just a quick way to access stuff, and not actually a storage location. I've seen many lose applications and cause themselves much inconvenience this way.

System Preferences.app is in /Applications.

Ahh, yes, so it is. I don't usually go there for it, and was making the assumption it would be in Utilities. Sorry for that!