Base SDK missing error

I am seeing "Base SDK missing" er开发者_如何学Cror for some of the projects I open in XCODE. Few days back I upgraded my XCODE version to 3.2.5 and I am not able to see IOS SDK 4.0 in it. Could you please suggest me what should I do?

Go to the project settings, and select as "Base SDK" "iOS 4.2" or whatever is the current version. If you want compatibility to older iOS versions in the AppStore, select as "iOS Deployment Target" the desired lowest version, but be careful when using apis which aren't aren't available in that version (respondsToSelector: is your friend).

Base SDK can be set in your Project Info under the General tab.