Also bear in mind that taxis in MAD are very reasonably priced. Personally, if you have an appointment and you are totally unfamiliar with the city and/or carrying luggage with you, I'd just jump in a cab. During daytime it should cost you around 20 (absolute max 25) Euros from the airport and under 15 from central Madrid. Sure, it's a lot more than the 64 cents the metro ticket costs (if you buy 10 together- single price 1 Euro), but IMHO it's not a lot of money in the grand scheme of things!