Hey there BritMe,
If I understand correctly you want to be able to send iMessages that you get on your phone from your computer as well. For iMessage, the minimum OS requirement is 10.8 or later.
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage
Your iPhone telephone number is linked to your Apple ID when you sign in to FaceTime or iMessage. This allows you to use your iPhone number with FaceTime and iMessage on other iOS devices.
- This feature isn't available on iPod touch (3rd generation) and iPad (1st generation).
- This feature is available with OS X Mountain Lion v10.8.2 or later.
To be able to send SMS or MMS messages from your computer through your phone using Messages, you would need to have Mac OS X Yosemite installed, since that is part of Continuity feature called Hand Off available between an iOS 8.1 device, and Mac OS X Yosemite but your computer does not meet the requirements for that OS.
Connect your iPhone, iPad, iPod touch, and Mac using Continuity
Handoff and Instant Hotspot are supported by the following Mac models, and require OS X Yosemite:
- MacBook Air (Mid 2012 and later)
- MacBook Pro (Mid 2012 and later)
- iMac (Late 2012 and later)
- Mac mini (Late 2012 and later)
- Mac Pro (Late 2013)
Thank you for using Apple Support Communities.
All the very best,
Sterling