From a fellow BAExpater the bus to take is:
129 Plaza 'a la plata por autopista y camino centenario'. Make sure it says Express. Organize with the owner Adrian before hand, call him when you arrive and he will pick you up at the bus stop. It will take 50-60 min by bus only 25-30 min by car or remise service.
Thank you, Santi for bringing me along last weekend. It was a wonderful experience. I had never really ridden before and the instructor taught me basic skills in just 2 hours. I will definitely go back there in the future.