If you are talking about a Pilgrim Credential, it is my understanding that you can obtain one from the Pamplona Alberque or from the University on the outskirts toward Cizar Menor. Depending on when you depart, you might also consider obtaining one from a pilgrim association and giving them your support.
I got my credential this past May for 1 Euro from the Archbishop's Palace, which is just around the left-hand corner from the Cathedral.
I must add also that I got hopelessly lost trying to find the Cathedral. Must have walked 12 km all around the city and surrounds that day. And it was the only day I got blisters: I hadn't prepared my feet, because I didn't plan to walk that far. I was using the city map in the Brierley guide, but for some reason I just couldn't find the Cathedral and actually ended up in a suburb not even on the map! By that time, my rudimentary Spanish skills had evaporated in the 32 degree C weather. Finally an English-speaking Spanish businessman came to my rescue and drew exact directions and landmarks out for me.
Maybe it was due to the jet lag (it was my first full day there) combined with the heat. But, though lovely, but I found Pamplona's streets mucho confusing. So if your Spanish is not good, maybe print a more detailed map off the internet before you go.
Re: Credencial in the "Casa Paderborn" at Pamplona
You can also obtain your Credencial at the both refuges at Pamplona:
In the Albergue de Peregrinos "Casa Paderborn"
or the municipal Refuge in the Calle Compana.