Belinda answered directly and everything went perfect. Her boyfriend recieved us at the apartment, and explained us everything. He also gave us a maps-print with the nearest supermarkts. The appartment has a very good location, 5 minutes away from the next metro station, and 3 blocks away from the train station "Atocha". The appartment is in the 4th floor, but there is no elevator (maybe important for old people...).
The appartment was very clean, and we had everything we needed (Also internet). The bathroom is maybe a little small, but it should not be a problem if you stay there for a few days. I had no problem with it.

One advice: be very careful with the taxis in Madrid. Because we had a lot of baggage we took a taxi from Atocha Train Station to the apartment (300 meters) and we had to pay 20 €, because he drived a long way... You should be careful with it, and always ask for the price before you get in. I just supposed it wouldn't cost so much, so thats why I didn't ask for the price. My mistake.