Does anyone know how to use this App

The Help menu "Messages Help" will "help" you setup and use your Messages app.

I think you have to set up your Apple account to make Messages work for you.

To: gets the phone number that you send your message to.
enter the message. Send it.

 

Without an iPhone you won't be able to use it for texting. You can use it along with your AppleID to send messages to people with iPads, iPod Touches and iPhones, though.

 

Ah - so, just to confirm - it is not possible to text from OS X Messages app unless you have configured your Mac with an iPhone/iOS device, and that you also need connection to that iPhone to be able to send messages to an Android phone.
And that's the only way to message from OS X to Android (via an iPhone/iOS device) (?)

 

You can also use it for messaging with Google Hangouts users, I do it all the time. You just have to add your Google account in the Internet Accounts section of system preferences. I have both a Google (Gmail) account and a Google Apps account set up on mine.

 

You could use Whats App, Facebook messenger etc

But if you need to send SMS you need to setup Text Msg forwarding on an iPhone

 

I may be restating the obvious . . . my apologies if I am.

Messages on the Mac is dependent on YOU signing in with your AppleID/iCloudID. Two discriptions that mean the same thing. In other words, you Apple Account. Once in, you can send text messages to any other Apple user who has signed in with THEIR AppleID/iCloudID. Your transmissions will appear in a BLUE bubble if you're sending to another Apple account that is signed into THEIR account. If it goes in GREEN, then the receiving party is not signed into THEIR account. With out the added benefit of an iPhone, you cannot send text to a non-apple user from your Mac using Messages. To send to those folk, pick up your iPhone and tap away. Have I missed anything?

 

I guess the OP has figured it out sometime in the last 2yrs ;-)