I'm not sure where you are located or exactly when you will depart for Pamplona but an excellent option is to use this website:
http://www.simcardspain.es/en/ . I have purchased my loaded, activated sim card from them three times and love this service. It is mailed directly to my home in Canada. I simply insert it into my unlocked iPhone and it is ready to go. If/when my credits are low I can easily go online to the same website and top it up whenever is convenient to me. The only caveat I have is that it usually take a month or so to arrive so if time is a worry this might not be the best option for you. If that is the case I'd recommend just googling the location of a Moviestar store in Pamplona and getting the address and business hours. I know there is a store directly on the Camino as you leave the old town. I'm sure Vodaphone, Orange and others are available but I only have first-hand knowledge of the Moviestar store. After that any of a thousand small stores across the country are available to add credits to your phone. Keep in mind that virtually every bar and Albergue has wifi so the necessity of a phone is probably limited to emergencies in remote areas. You know, that is unless you feel the need to post a selfie from Cruz de Ferro or some such place!