This is just from personal experience -- It usually opens after 9:30am on the weekends, busy days and every day during the summer. If it is an uncrowded weekday it generally is closed but that doesn't mean you can't enter there. If you do they will give you directions to the structure.
I got directed to the structure. Was there just after 8am this morning. Didn't have that bad of a space though in the structure since I have a disabled placard. I prefer Timon, but it wasn't to bad. Now, when I left at 10:30am, it was packed on the lower level!
Thanks for the info!