Using o2 unlocked iphone in Spain

I have had my iPhone unlocked by o2 so I can use my phone in Spain with a Movistar sim as my employer has moved me to Spain for 6 months. Problem is with the Movistar sim inserted I loose the 3g and the Cellular Data Network tab is missing from the settings menu all though it is there with the o2 sim re inserted. Is it possible to manually input the settings? If so how do you do it?