Hey guys, I managed to get everything worked finally. Someone on GameFaqs posted saying he was having the same problem and well, it worked for me.
Basically, I had received an e-mail with a europeean code, the one that starts with RAAE, and to register I needed the one with RAAU.
In short, all we need to do is go to preorder.na.finalfantasyxiv.com and keep trying with our 18 digit code and eventually it should work. Instead of an e-mail, you should be transferred to another page telling your code and the link to register. That's it.
After I read the instructions of the gamefaqs guy, it didn't take me more than 5 minutes until it worked.
So, if someone was still unsure if they should/can type in their code, it is working now. Hope this helps anyone here.