Get your tickets and join the fans at a game at Cuscatlan Park. If there's nothing on while you're there, you might like to check if the stadium offers a guided tour of the grounds.
You're spoiled for sightseeing choice in San Salvador. Cuscatlan Park is a good place to start here, but there are plenty of other worthwhile places to visit. Cuscatlán Stadium, National Theater, and Plaza Merliot are not far from the sporting facility and will fill in your day nicely.