San Sebastian Airport (as opposed to its bus station) is not in San Sebastian, but on the outskirts of Irun, the start of the Norte. From the airport entrance, you simply turn left and you can walk to the Norte path in 15 minutes. It's just over a kilometer from the airport. I've pasted in a screenshot from Mapy.cz. At the intersection of the Norte, turn right and it's a further 15-16 kilometers to Pasajes, depending on which route you follow. If you prefer to not walk to Pasajes, you can pick up a local bus (they run frequently) from right near the airport directly to Pasejes. Consult Google maps public transit option for exact details.
Btw, if you do decide to walk to Pasejes, the Purgatorio alternative, which starts at the Sanctuary of the Virgin of Guadalupe, about 3 k after you will intersect the Norte, offers one of the most spectacular sustained panoramas of any of the major caminos. It does require a bit of an initial climb, however.