Looks like AA68 usually departs from the E gates in MIA, so the Admirals Club in E makes sense. There is no arrivals lounge in MAD available so nothing on the inbound for you.
Outbound, you'll want to look for Iberia's "Sala VIP" in Madrid's Terminal 4-S; after the underground tram ride. It is pretty much in the mid-section of the terminal and it's official name is Sala Velásquez. It is a very comfortable lounge with good showers, nice variety of beverages but very light snacks only, like nuts, muffins, chips, etc. If you are a light eater in the AM, you should be fine having breakfast there before you get a full lunch service on AA69.