If you're arriving to Pamplona by train, and your apartment or hotel is in the heart of the festival, you'll want to take a cab or a bus into the Casco Viejo, or Old Quarter, where the festival takes place.
Cabs are usually available, but the public bus is also an easy and reliable option if you'd like to save a bit of cash. When you step outside the train station, you'll immediately see a bus stop, as shown in the picture on this blog post.
If you have cell service, you can do a quick search on Google Maps to get the most direct bus path from the train station to your hotel or apartment. Otherwise, wait for either Bus L09 or L07, then hop on the bus and pay the fare with euro coins. You'll be on the bus for about 15-20 minutes as you head toward the Old Quarter.
You can exit at any of several stops, including the Paseo Sarasate bus stop, to be a short walk 3-5 minute walk into the Old Quarter where the festival takes place. If you'd like to head to the main square, ask a local for directions to "Plaza del Castillo" when you get off the bus.
Assuming you've bought an International or Spanish SIM card/data plan, you can follow your bus route and walking route directly on Google Maps.