Centennial in Indiana? Where do you live? The EDGE icon will appear. That's it. It should work straight... unless your SIM has had its access to data blocked. In which case, you need to take it to a Centennial place and get one that doesn't have the block or have yours rewritten (I have no idea if that's possible)
I'm thinking about Centennial. That, or wait two years for Verizon to finish going GSM.
I unlocked mine the exact same way. As president of NHS, I had the authority to take a lesser member's SIM and pop it into my iPhone to make sure it had worked. It did. The EDGE logo appeared, and "Centen..." was in the carrier bar. You might be in a place without EDGE service.