Actually there is no need to Enter an "Unlock Code" on Apple iPhone, all the process is done via iTunes once you receive from us the confirmation email of Unlock. The most important part is in case you "Jailbreak" your iPhone in past to unlock it, that you remove all application related to that. Once your iPhone is clean from any Jailbreak Application please follow these steps :
1 - Install the last version of iTunes
2 - Make sure your iPhone is updated with the lastest version of Firmware (iOS)
3 - Swiftch on your iPhone with an Not Accepted SIM Card
4 - Connect your iPhone to your computer using the Original USB Cable
5 - Launch iTunes & wait until iTunes detect your iPhone
6 - Now disconnect your iPhone and reconnect it after 10 seconds.
7 - Your iPhone is now Unlocked.
PS: in Case your iPhone is not unlocked, just do "Restore" !
iOS7 Update: How does Activation Lock work?
When you enable Find My iPhone on your iPhone, iPad, or iPod touch running iOS 7, the Apple ID and password you use for iCloud is securely stored on Apple’s activation servers and linked to your device. From that point on, your password will be required before anyone can turn off Find My iPhone on your device, erase your device, or reactivate and use your device.
Activation Lock is enabled automatically when you turn on Find My iPhone in iOS 7, or when you upgrade to iOS 7 from a previous version of iOS that has Find My iPhone already turned on.
To turn on Find My iPhone on your device:
Go to Settings.
Tap iCloud.
Sign in with your Apple ID, if necessary.
Turn on Find My iPhone.
Prior to unlocking a phone, make sure the previous owner of the unit has turned off findmyphone and erased all contents & settings.
Request UnAvailable***