Hi Rita,
You'll need to get into town first and then take the 6pm bus to Roncesvalles (different bus company, different direction). The buses to the city run about every 30 minutes.
From Pamplona to Roncesvalles: Mon-Fri at ± 18h00 (+ 10am 1 July to 31 August) Sat at 9h30 (Not Sundays) ± €6
If you don't want to wait until 6pm you can share a taxi - also from the bus station.