MonoTouch error: "Apple iPhone SDK not installed" after updating to iOS 5.1
Have you just updated your devices to iOS 5.1? or
Have you installed the new Xcode 4.3.1 from the AppStore?
That's where the iOS 5.1 SDK is available and MonoDevelop 188.8.131.52 should pick it up automatically.
From the MonoDevelop menu, select About Mono Develop , select the Release Notes tab, and see if anything is missing (or just looks wrong) on the list. You can modify your question to include this information (up to the Loaded Assemblies section ).
source to share