以添加SDK5.1为例:
首先 下载xcode4.3 双击dmg文件打开 右键xcode图标浏览里面的内容 拷贝出上面所说的文件夹 过程如下：
1. 复制 文件夹 Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/5.1 (9B176)
到 系统文件夹 Developer/Platformes/iPhoneOS.platform/DeviceSupport
2. 复制 文件夹 Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk
到 系统文件夹 Developer/Platformes/iPhoneOS.platform/Developer/SDKs
3. 复制 文件夹 Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.1.sdk 到
系统文件夹 Developer/Platformes/iPhoneSimulator.platform/Developer/SDKs/
4. 复制 文件 Contents/Developer/Platforms/iPhoneOS.platform/version.plist 复盖
Developer/Platforms/iPhoneOS.platform/version.plist【记得备份】
5. 复制 文件Contents/Developer/Platforms/iPhoneSimulator.platform/version.plist 复盖
Developer/Platforms/iPhoneSimulator.platform/version.plist【记得备份】
6. 复制 Contents/Developer/Documentation/DocSets【所有文件】【到】Developer/Documentation/DocSets
重起 XCode 完成！