HELP!! I have tried everything to jailbreak my iphone 3g and nothing works?

[Elizabeth]

I just bought an Iphone 3g from a man that didn't need it any more before he got an Iphone 4g. My husband and i have tried EVERYTHING to jailbreak it. I read on like that if it is an MC it wont work so i check mine and it is MB702LL, and it is firmware is 4.1. We have tried downgrading the firmware, we have walked about 20 you tube videos and followed them step by step but it wont go to DFU mode. We have restored it about 10-15 times and nothing will work. My husband is good with computers so it is weird that he has not been able to figure it out, and we really need some help... PLEASE