Probably asked a thousand times so sorry, but what does it take for the Norman culture to appear? As a Norse, I took Rouen 20 years ago, I often put my Intendant on Implant tribe and nope, it's still French.
So I'm really just not lucky (especially since I now have two french counties).
Also, Norman Catholics can still raid, unless Conclave patches fix that.
As one of the above posters mentioned already, it's rather random but the only requirement is that you're Norse and then own a French/Breton/Occitian province. A lot of the time if you start off as say Haesteinn of Nantes, you'll tend to become Norman pretty darn quick. I can recall a multiplayer game I once had with a friend, and as a Norseman he literally became Norman a day later after doing a Conquest of Rennes.
On a side note, Norman is technically a naval raiding culture in the same way Danish, Swedish, or Norwegian is but not Norse. The catch is that Norse can always raid regardless of religion or government, whereas all of the others are bound by either or. For example, a Norman Catholic Feudal can't raid, but a Norman Germanic Feudal or Norman Catholic Tribal could. Suffice to say, I'm rather fond of becoming a Norse Christian because of this rather than reforming the faith.