If I'm not mistaken there are only 2 Priority Pass Lounges at MIA - one in Terminal G and one in Terminal J - neither is very convenient to Terminal D where you AA flight to STX will depart from. If I had to choose, I'd pick the lounge in Terminal G as it is a bit closer to your connecting gate.
As for navigating the airport - on the way down DO NOT leave the secure area - you can use "inside corridors" to walk between Terminals. All the shops and restaurants are inside the secure area. Go here for a list of places and locations:
http://www.shopmiamiairport.comIf you like Sushi try Sushi Maki near Gate 29 in Terminal D - it's pretty good.
