I don't have SMS center number either, but mine is editable by finger selecting it. If you can edit it enter +12063130004. which is TMobiles center number.
I get to mine by going to Settings, swipe to the right, Messaging, select SMS center number.
edit: I just looked at the SMS center number again and found that with Data connection off it is visible, with Data connection on it is not. Hmmm...
Good Luck,
Ed