Did you turn off cellular data through the settings? If you did then MMS won't work since sms does require a small amount of "data". If you arent subscribed to a data plan, like me, then toggle on the cellular data option, and just go to unlockit.co.nz and disable data that way.
^ In other words, you'll be able to send MMS and not be charged for any data usage if you disable data through the alternative method I linked above. Turning off cellular data on your phone blocks ALL data in and out, sms (mms) messaging uses "sms" data thats ONLY charged to your sms plan and not to your data plan