Cannot find `aapt.exe`. Please install the Android SDK Build-tools package

Google has updated their SDK tools ("Android SDK Tools" Rev. 23) in a way that also requires installing the latest "Android SDK Build-tools" package, and the Android SDK Manager does not enforce this requirement.

Fix: once you have installed "Android SDK Tools" Rev. 23 or higher, you must also install "Android SDK Platform-tools" Rev. 20 or higher and "Android SDK Build-tools" Rev 19.1.0 or higher (Rev. 20 recommended). This is necessary because Google moved some of the executables that were previously located in the "Android SDK Tools" package into the "Android SDK Build-tools" package.

After this step your Android SDK Manager should show up-to-date versions for all three packages:

Error messages

Xamarin Studio on Mac

error XA5205: Cannot find aapt. Please install the Android SDK Build-tools package with the /Users/username/Library/Developer/Xamarin/android-sdk-macosx/tools/android program.

Visual Studio on Windows

Cannot find aapt.exe. Please install the Android SDK Build-tools package with the C:\Users\Username\AppData\Local\Android\android-sdk\tools\android.bat program.

http://forums.xamarin.com/discussion/20505/error-xa5205-cannot-find-aapt-exe-please-install-the-android-sdk-build-tools-package