Changing Carrier on Jailbroken iPhone 4 to T-Mobile, No "network" option

I've jailbroken my iPhone 4 and what I'm reading says to change from AT&T to T-Mobile, I need to go into Settings>General>Network then input all sorts of info. My problem is when I go into General, there is no Network option. If I go into About, there is a place that says "Network" then "Not Available." I see: About Software Usage Cellular VPN iTunes Wi-Fi Sync Sporlight Search Auto-Lock Passcode Lock Restrictions etc.