I'm trying to register to order RENFE tickets online.
The site is confusing:
It wants me to enter somthing labeled, "N.I.F"
what's a NIF?
there isn't a help button and you have to already be registered to email them.
Also, I assume that when I enter my date of birth it's in day/month/year format. iss that correct?

I had the same problem. I finally put the first letter and as many numbers of my passport that would fit into the space and it accepted my entry and I was able to reserve my tickets. Hopefully I'll have no problem picking them up - the site said first time users have to pick their tickets up at the station.
Hopefull someone will have a more accurate answer than mine.
Good luck,
Robert

N.I.F. is like your ID number. You are fine doing as biskraboy did, enter as many numbers from your passport as it fits.
The important thing you are going to need is your credit card, make sure you take it with you when you go to pick up your tickets, it has to be the same one that you use to purchase online.
Also, if you want to avoid lines pick up your tickets at the RENFE counter inside the airport (I think is in the main lobby. Chances are you'll have to wait a long time at the train station as it's always crowded.
I did all this a couple years ago and it worked out really well.

This topic has already been discussed on MadridMan.
Please see this thread.
http://www.madridman.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=3;t=001652#000000

I make no guarantees of this, but I´ve just bought RENFE tickets over the internet for the first time (their system doesn´t accept UK postcodes either, incidentally). I took the option to print the tickets at home and that was it - no need to queue to pick anything up, at the barrier we just showed the paper tickets I had printed and got on the train. It´s not what they tell you to do on their website, for all I know it might be a glitch in the programming for the booking system, but it worked for us.