What version of the software are you using? A lot of brand new 8520s shipped with 4.6 and 8530s shipped with 5.0, based on my experience.
These instructions are for v188.8.131.526 (Telcel MX), should be similar for any 5.0
BB key -> Manage Connections (little antenna icon) -> Mobile Network Options
Then scroll down to Network Selection Mode, change to Manual, let it scan, then it will show a list of available networks, then select the one you want.
If the phone is defaulting to Rogers, then it's possible that it's not reading the SIM properly, so remove and reinstall the SIM first and see if that fixes it, it's happened a couple of times with my unlocked 8520 (Which is a Telcel phone that was unlocked but still being used with Telcel, and a couple of times it has defaulted to Movistar)