'Las Americas?' Do you mean '¿Avenida de America?' There's nothing between Madrid's aeropuerto de Barajas and the Avda, de America multimodal transportation Center other than Autopista de Barajas, a limited access expressway.
If biskraboy takes the Metro, he should continue on linea 8 to Nuevos Ministerios and then take a taxi to his destination (or, I believe he may be within walking distance to hostal Adriano).