It is better you dont try any softwares to unlock because it may spread viruses to your device and corrupt it sometimes. You can try to unlock your mobile using codes at TheUnlockArena.com very easily.
I think the MEP code is wrong so only nothing is working. You can get correct unlock code from TheUnlockSpot.com very easily. I got my codes from there and unlocked my mobile sucessfully. You too try it there.
Do not get free codes as they may be wrong and if you use softwares also they wont unlock your mobile properly. Always unlocking using codes is the best solution. You can get these codes at SuperUnlockCodes for a cheap price. Check it out.
Yes if the iphone has a network other than US Cellular you need to unlock it to use your SIM. Unlocking process is very simple. Check out Superunlockcodes.com for obtaining the unlock codes. Follow the instructions at MobileUnlockGuide which is very simple.