Sorry, but without an iPhone you are limited to texting using iMessage only to other iOS users. Did you post this question a few days ago? If you did, I let you (or whoever the other person) know this isn't possible without a companion iPhone.
iPad with celllular is for data only. iPad does not have a cellular radio like a cell phone, therefore it cannot text using cellular radio for SMS/MMS.
So, your options are as follows:
- Purchase an iPhone (which would make so many things better in terms of syncing data across devices)
- Use a 3rd party texting service (it won't let you use your cell number to text to or from)
- Use your 14 day no questions asked return policy and get your money back