How to Send instant Messages as SMS
How do I send Im as text message form my computer?
MacBook Pro (Retina, 15-inch, Early 2013), OS X Yosemite (10.10.5)
Follow the instructions in this support article, under the heading
To use Continuity for SMS and MMS with your iPhone and your Mac, iPad or iPod touch
Note that if you've enabled two-step verification for your Apple ID, you must generate an application-specific password and enter it when you sign in to iMessage.