How do I get rid of this question mark shown in Launchpad?

That is because all Apps that are in the Applications folder get an Icon in the Launchpad area but as with Lion, and now Mt Lion, once you install the OS from that download, that is placed in the Applications folder, the Installer DELETES those files but it does not remove the Icon from the Launchpad.

Just DRAG it to the Trash can. You may have to hold down the Option or Command key (I forget which or if you even need to) while dragging it to the trash can.

Thank you so much. It was driving me crazy. It's command key.