You will need to go to your iPad settings/iMessage and activate it by switching it on. It will then ask for you to sign in with your Apple ID and your password to verify it.
Once you have done that, you can iMessage from your iPad to anyone that also has an idevice.
Have you already signed in? Please tell us what you have done. It is difficult to discern what you have done already based in your posts. In addition, just checking my iPhone...I can scroll down to get to SMS and mms. I do not have that feature on my iPad/wifi.
The person you are sending your iMessage to....do they have an idevice as well? It could be that they may not and are using an android phone. Also, they may have their iMessage setting turned off on their idevice.