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I had the same issue. A window popped up on my phone asking me to enter a code which would not show up on my mac. I turned off iMessage through settings > messages > iMessage. Closed messages on my mac as well. and then i turned them both on again. At first i couldn't even see the setting for text message forwarding, but then my mac started to (seemingly) randomly throw windows at me. Asking me if i wanted to use my mac to send text messages. So i accepted. Then the mysterious number popped up, and the window on my phone was back again. When i entered the code, it worked. So now i can use iMessage on my mac to send text messages. I hope this helps, i kind off found the solution by accident, so i don't know if this is the best way to go about it.
 
On your iOS device there is now a Settings > Messages > Text Message Forwarding section.

Iphone 6, iOS 8.2

I don't have this option in settings. Is it because I don't have Yosemite installed? How would my phone know this???
 
Iphone 6, iOS 8.2

I don't have this option in settings. Is it because I don't have Yosemite installed? How would my phone know this???

Well your phone does have to connect to your computer over Bluetooth LE for the Handoffs feature to work. And it specifically lists compatible computers under the Text Message Forwarding menu.
 
I had the same problem with it not giving me the code

All I did to fix it was I selected my email as well as my phone number under "send & receive" on all of my devices. After I did that I went back to the "text message forwarding" on my phone and then I turned on each device and the codes popped up on all of them.
 
In your ipad settings -> Messages -> send & receive -> make sure your phone number is checked. The code should appear when you toggle the text forwarding toggle on your iphone.
 
Having the setting completely disappear just because your other settings aren't configured correctly is a bit cuckoo. They should have had it just gray out, with text underneath explaining why.
 
Obviously you need both your iPhone and iPad/Mac to be on the same network whilst setting this up.. it does a few minutes to register..
You also MUST have your iMessage email address and mobile phone active in messages on all devices before it will work.

Patience is a virtue :)
 
I don't see that option anywhere.

Make sure iMessage is enabled and activated, and you're running ios 8.1.

As far as the code not appearing on iPad/iMac.
First, I tried the enabling emailadresses for imessage suggestion, but only that didn't make it work.

What finally made it work for me, was turning iMessage off and on again, on my iPhone.
That made my iMac and iPhone come up with a pop-up asking if i'd like to use the mobile phone number of my iPhone.

After that, i retried enabling sms relay for both iPad and iMac and the codes turned up fine.

These little sync problems can be so intensely frustrating.
Hope my experience helps anyone.
 
I tried everything posted here but nothing worked. Reset iMessage a heap of times on all devices. Made sure they were all setup with the same phone number and email address and I still wasn't getting any codes.

In the end, I reset my iPhone 6 by hold the home and sleep/wake button until the apple logo appeared and then disappeared again. Only then did I let go of the buttons. The iPhone rebooted and only then did I get the code on my Macbook and iPad.
 
I tried everything posted here but nothing worked. Reset iMessage a heap of times on all devices. Made sure they were all setup with the same phone number and email address and I still wasn't getting any codes.

In the end, I reset my iPhone 6 by hold the home and sleep/wake button until the apple logo appeared and then disappeared again. Only then did I let go of the buttons. The iPhone rebooted and only then did I get the code on my Macbook and iPad.

This may be a stupid question but what the hell. I have the same problem you encountered. the code shows up on the iPad Air and I put it on the iPhone 6 plus. However, nothing shows up on my Macbook pro with Yosemite X installed.

If I do as you did and hold the home button down and the sleep/awake at the same time to reset, will I lose all my custom settings and data on the phone (like ringtones, text tones, messages, emails, camera roll)?

I've tried everything to get my Macbook to work (iMessage settings has 4 tel number and emails check off and on same wireless network).
 
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