如果我们在网上下载的低版本开发的iOS源代码，而我们的iOS SDK又是高版本的时候，就会有Base SDK Missing的问题缠身，icech在网上搜集了两个解决办法：
方法1：老外的方法，网上流传广泛。IPhone Base SDK missing
1. Load your project
2. From the menu, select Project > Edit Project Settings...
3. Under Architecture > Base SDK, choose one of the available device options: iPhone 3.2 or iPhone 4.0. If you are iPhone-only, 4.0 is the way to go.
4. Close that window.
5. From the menu, select Project > Edit Active Target "YourTarget"
6. Under Architecture > Base SDK, choose one of the available device or simulator options: iPhone 3.2 or iPhone 4.0.
7. If you want to target previous iOS versions, then in that same window, under Deployment > iPhone OS Deployment Target, select the lowest version you want to support. Note that support for 2.x versions through the app store is deprecated. See the "Readiness Checklist" quote above.
方法2：看起来和方法1没什么区别，呵呵
解决办法就是Project->Edit Project Settings->Build标签页中的将默认的Base SDK从3.2版本选择成现有的iOS SDK 4.2。可这样依旧不能解决问题。关键还有一个步骤，Project->Set Active Target，选择成你现在的工程。尽管你会看到，Project->Set Active Target中只有你现在工程的名称，并且已经打钩了，但你仍然要用鼠标点击一下。