From Cydia, Manage, Packages, SMS GV Extension, Modify, Remove. You will use the standard Messages app that comes with iOS 5.0 and up. The very same that you use to send iMessages.
Okay. This is going to be as clear as I can possibly make it.
Open iFile. Or SSH into your iPad, (iFile will be easier).
Navigate to "/". Just press back until it won't let you go any further back.
Tap on System, then Library, then CoreServices, and finally SpringBoard.app
Scroll through this folder (Springboard.app) until you find the K section. Now find the .plist file titled something like K**AP.plist (The * will be replaced by numbers)
Open this file and view with the text editor (I forget the options but one will open up a chart styled window and one will open text. We want the one that opens text)
Now we are going to add a value. Find the section called "capabilities". It should be near the top and have things under it like Accessibility true and ect.
Now follow the same spacing and anywhere within the capabilities section put an enter between a <true/> argument and then next <key> line.
Enter the following; And remember to space this such as everything above and below it.
Save and respring. Wa-lah, your iPad now will send texts over your carrier as long as it isn't a data only sim card.