Make sure your status is Online in the Apple Messages app
Click on the new message icon, click on the “+” button from the new screen. Select “Buddies” from the left side menu and you’ll see your Android contact list. You can click on any of your Android contacts and start a new SMS conversation with them.
You can also click on “Window” from the Messages app top menu bar (while the Messages app is active) and click Buddies. This will show you all your Android contacts as seen below.