In case anyone is still trying to figure this out - cause I had the same issue...the problem is the APN settings with the carrier. I was able to send MMS messages before, i've only had the issue in the past 3-4 days where it was giving me an error message saying that I needed to turn on MMS messaging, but I had no issues last week and I didn't even have that option in my phone settings for turning on MMS so I didn't know what was going on. I had friends trying to text me pictures too and I never got them, and they never got error messages so they assumed I had received their text messages, which I didn't. I started thinking maybe it was that Wind Mobile, who i'm with, recently changed their name to Freedom Mobile and maybe it caused issues, so I called them. Super awesome agent who helped me and now my phone is working again and I have the MMS button where it's supposed to be. I'm assuming there was some kind of update that happened and it messed up the APN settings or something - I didn't set it up myself in the first place, so i'm not sure, but anyways, anyone who is with Wind/Freedom Mobile, here is what you need to do:
(I have an Iphone 6S...i don't know if this will work with other Iphones...if it doesn't then just call them and ask them what the settings should be)
1) Go to your settings, select "Cellular", then select "Celullar Data Options", then "Cellular Data Network"
2) Under the MMS heading, enter the following information:
APN: MMS.windmobile.ca
MMSC: http://mms.windmobile.ca
MMS Proxy: 74.115.197.70:8080
3) Power off your phone, wait at least 30 seconds, then turn your phone back on.
That's it!! You should be able to send MMS messages by doing that. The rep had me test it out after restarting my phone (she called me back 2-3 minutes later to double check), by texting a friend who had an android, to see if it worked and it did. On top of that, now when I go in my settings, that MMS on/off button that was missing before is now there in my settings. 🙂 Here is a screen shot of the above too, just cause i'm so happy my picture messaging is working that I want to post a picture. Hahaha. Thanks to this forum because if it wasn't for a previous poster, I wouldn't have known to call my carrier to get this fixed, I was going to go to the apple store to get someone to check my phone, thinking it was an Iphone issue. This was way easier. Hopefully this helps someone else too!! Happy New Year everyone!! 🙂