It's near the park. It's signposted but hard to see.
As you drive out the main park entrance, turn right and follow the road. There you should see the 'Chained Oak bed and Breakfast'. Park around there and by the driveway there's a little wooden sign pointing down a dirt track into the forest saying 'Chained Oak' walk down the path for a couple of minutes and it will be on your right. You can't miss it really.
As you exit the park turn LEFT (not right), the Chained Oak B&B is on the right. To the left of the B&B is a mud path leading down. Follow that for about 400 yards and the tree is on the right. Can't miss it.