If your SIM card is PIN locked, the network unlock code is useless. The network unlock code allows a phone that is restricted to a single carrier to be unlocked for use in other carrier's networks.
The SIM PIN is a personal identification number that is used to unlock the SIM card in order to register it to the network. This means that the phone will not connect to the network before a SIM is unlocked. Usually (at least around here) a new SIM has a simple PIN, like 0000 or 1234, and the user can change this. However, the carrier doesn't know your PIN, so if you forget it it can't be reset from the carrier directly.
If you happen to forget your PIN, the only way to unlock the SIM is to enter a wrong PIN enough many times (3 or 5 usually), and after this the SIM asks for the PUK (Personal Unlock Key) code, which is sometimes referred to as PUK1.
The PUK code can be obtained from the carrier, after they verify you are the rightful owner of the SIM and contract. If the SIM is a prepaid, the PUK code is sometimes printed in the card holder. After you get the PUK, enter it into your phone and enter a new PIN. After this, you can unlock the SIM with the new PIN you specified.
Note: DO NOT try to guess the PUK code yourself, if you enter it wrong three times, the SIM will be permanently locked and you'll have to get a new SIM from your carrier.