How to send SMS imessages from an iPad

You have to use text message forwarding form the iPhone in order to send SMS Messges on the iPad.

Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

To use Continuity for SMS and MMS with your iPhone and your Mac, iPad or iPod touch

Your iPhone, iPad, and iPod touch need to use iOS 8.1, and your Mac needs to use OS X Yosemite.

1. Sign in to iMessage on your iPhone, your other iOS devices, and your Mac using the same Apple ID.
2. On your iPhone: Go to Settings > Messages > Send & Receive > You Can Be Reached By, and add a check to both your phone number and email address. Then, go to Settings > Messages > Text Message Forwarding, and enable the device(s) you would like to forward messages to.
3. Your Mac, iPad, or iPod touch will display a code. Enter this code on your iPhone to verify the SMS feature.

Thanks Demo...I just added a new phone to my plan and totally forgot about that setting. 😎

I was getting a "Failure to send" advisory after attempting to send a text to a non-iMessage user. I had done this in the past and I was wondering why the "Send as SMS" disappeared from the iPad>Settings>Messages> setting. 😊

You're welcome. It's been a while since I have had to add a device, so I had to research it again myself rather than explain it from memory.