How can I send an SMS message from my Mac?
This is called Text Message Forwarding and requires iOS 8.1 or above. Make sure both the OS X Messages App and your iPhone have the same Apple ID enabled. Then, do the following:


On your iPhone, tap Settings > Messages.
Tap Text Message Forwarding.
Enable your Mac in the list of devices.
On your Mac, open Messages.
A six-digit activation code appears.
Enter this code on your iPhone, then tap Allow.


That should just about do it.