Re no tethering option under Settings general network iPhone 3G

I am using iphone 3g with 3.1.2 firmware build 7d11.i have jailbroken my phone and also unlocked it.but under the settings>general>network iphone tethering option is not available.what can i do to get that for tethering my phone.i have also tried this link below but nothing helped,the tethering option is still not available.HELP PLEASE.http://help.benm.at/help.php Iphone iPhone OS 3.1.2