You will need to contact Virgin Mobile US and ask them for an unlock code.
Alternatively you could do it yourself by rooting the phone using ADB. However no one on tom's will help you do this as it's against the rules. If you choose to do this, remember it voids the phones warranty and even the slightest mistake could turn it into a very expensive paperweight. You can find help at the XDA forums. http://forum.xda-developers.com/