The straight-forward way to add additional Simulators to XCode is via Preferences. XCode 4.5.1 comes preloaded with iPhone/iPad Simulators for SDK 6.0 and the option to download 5.0 and 5.1.
However other versions are getting little love. If you need to support clients on older versions of iOS this is a problem. Luckily, adding older Simulators to XCode is easy.
NOTE: XCode 4.5 has dropped armv6 support so can no longer be used to develop for iOS <4.3. Adding armv6 simulators such as iOS 4.2 to XCode 4.5 will have no effect, they will simply be ignored.