I do believe you but... I've switched realms several times and it will not open!!! So do you have to be a certain level to be able to go to the secreat shop?
You don't have to be a certain level. There are lights on the obelisks. When all of the lights are glowing, then the portal opens to the Secret Shop. (I believe it is something like every 10 minutes or so, that another light glows. And when they are all glowing, it opens.)
I think Prof. Greyrose meant that each realm is on its own timeframe. So by switching realms, you might find one already open, or very close to opening.