Xcode 'CodeSign error: code sign is required'

I've been working on an iPhone project with iOS 4.0. I just downloaded Xcode 3.2.4 with iOS SDK 4.1 so that I can work with the updated iOS. Upon opening the project in the udpated Xcode, I found that the target read 'Base SDK Missing'. I fixed that by following the instructions I've been working on an iPhone project with iOS