Google have made version 2.1 of the Android SDK available on their developer web site. If you have a previous version installed already, you can update it by running the Android SDK Manager.

On Windows, do this by running the android.bat file in the tools folder of the extracted SDK. On Mac open Terminal and navigate to the Tools folder, then run

./android

Check for available updates and select the option to install the 2.1 SDK.

Once installed, you will have the option of creating a new emulator image on the 2.1 platform:

For more information on how to configure the SDK on MacOS, read this article - http://blog.brightpointuk.co.uk/google-release-version-20-android-sdk