If you read the Add-on page more carefully you'll see this:
The Google APIs add-on is not yet available for download. For your convenience, the Google APIs add-on is available preinstalled in the Android SDK.
So you should already have the Google APIs add-on already if you've installed the Android SDK.
To check, run the android
command with no options at a command prompt to open the Android SDK and AVD Manager GUI. Check Installed Packages to confirm that Google APIs is listed:
If it's not listed use the Available Packages option to install it.
If you're using Eclipse, you also need to edit the Properties of your Project and select the correct Build Target: