How to find currently opened page in the navigation bar at the left in MS One Note? Suppose I have million pages.
Well you seem to have found yourself in a pickle. I have never needed to find the current page I am on since I usually have less that 50 or so. But I found a sure fire way to show you where the page is located. (I am using OneNote 2013, so this may not be viable for the version you have)
In the ribbon, go to "VIEW",
Then "Full Page View",
In the Upper right when you press the down arrow next to your notebook name the box here will always auto scroll to to current working page.
You may also search for content on a page and when it takes you there, you can open the drop down and it will be scrolled to the active page.