Hello, chrispynj.
Thank you for using Apple Support Communities.
From what I've read is that you're unable to send messages to Android users on your MacBook Pro. I got you covered and this can easily be explained. In order to send an SMS/MMS message you need a text-messaging plan, which is provided by a wireless carrier for cellphones. Contact your wireless carrier. Since the MacBook Pro isn't a cellular device, you won't be able to get a text message plan.
However, there is an alternative since you have an iPhone. You can set up text message forwarding that will allow you to be able to send those messages to your Android contacts. Check out the article on how to set this up: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac
Cheers!