why can't build windows phone project on unity3d 4.2

i already install windows8 64bit pro and visual studio 12 with windows phone sdk 8.0, but when i switch my unity project to windows 8 or windows store apps, the "Build" and "Build and Run" button was gray and disable. there have a tip "You have to use Windows 8 or greater for this build". how can i do ?? plz help me.