I fixed it so it worked

nav_game.py

@@ -2,3 +2,13 @@
 # ------------
 # By MWC Contributors
 # This class implements a simple game where a spaceship avoids asteroids.
+
+from retro.game import Game
+from spaceship import Spaceship
+from asteroid_spawner import AsteroidSpawner
+
+board_size = (25, 25)
+ship = Spaceship(board_size)
+spawner = AsteroidSpawner(board_size)
+game = Game([ship, spawner], {"score": 0}, board_size=board_size)
+game.play()