I don't think the Rain Forest Cafe takes ADR. At least on our last trip we tried making reservations at the one in DTD. They told us they do not accept reservations. Maybe they changed their policy.
Yes, The RainForest Cafe outside AK does take reservations but you have to call them directly not Disney. I think I got the number from Allears. I would definately call them the place was packed when we went and had to wait about 20 minutes even with a reservation. It was a fun place and my kids just had a blast there.
The previous posts were 50% correct. The RFC in DTD doesn't take ADR's. The RFC at AK does. I know, I have one for later this month. I would recommend an ADR for RFC, it's the only full serve restaurant in the Animal Kingdom. I did the ADR thru Disney Dining.
We had an ADR through Disney Dining for our trip over a week ago. It was a good thing, since people were lining up there for lunch! It is a nice, air-conditioned place to take a break for lunch.
The RFC at DD does take ADRs, but those you have to make through RFC directly!