The Rainforest Cafe at Downtown Disney make their own seating arrangements for guests. On the day of, it is walk up only.
You may make PS for the Rainforest Cafe at Animal Kingdom through the Disney Dining system, but not on the day of... in that case, you would have to make them directly through the restaurant. Also, the Rainforest Cafe at Animal Kingdom serves breakfast, but the last dining time available in the evening is at 6 pm.
Hope that helps!