Android SDK does not include error

I have installed Android SDK in my Windows. Its path is D:\Downloads\android-sdk-windows\android-sdk-windows. When I build Android apk, unity tell me to indicate the Android SDK root folder. After I set the root folder it warning that "Selected folder is not empty, Do you want to replace selected folder? Replacing it will overwrite its current contents." I select yes. The error message display : Android SDK does not include any platforms. But my android SDK has android-8 directory which is android 2.2 API. How can I set the SDK to make unity can build.