You or the previous owner need to ask the carrier to remove the PIN from that network. This is not unlocking it, just removing the PIN from the old account.
THEN, you will need to have the device unlocked.
Is it already? -->
How do I determine if my BlackBerry is "unlocked"? - BlackBerryFAQ
If not, then read -->
How do I unlock my BlackBerry? - BlackBerryFAQ