Unlocked Nexus S not recognizing Virgin Mobile SIM card

I unlocked my friend's Nexus S using Nexus Root Toolkit, and it unlocked properly. When it boots up, there's a small unlock symbol underneath the Nexus logo, but it won't recognize the SIM card or connect to a network. I went into Settings > Wireless and Networks > Mobile networks > Network operators and it showed VIRGIN, VIRGIN, and VIRGIN, but won't connect to any of them. Does anyone know why it won't connect? Did I miss a step somewhere along the way? I've tried rebooting and pulling the battery out many times but it hasn't helped. Update: In the settings it reads the IMEI and the phone number, so it's obviously reading the SIM card, but why won't it connect to the network?