does the tuition still show as available on your 2015 Notice of Assessment, or on your My Account information when you log in to the CRA website?

On the CRA website I have $4000+ under the column 'Amount available for carryover'

Do I enter that amount as unused tuition from previous years?

As long as you have available tuition credits, you are able to claim them on your tax return.  Your Notice of Assessment will let you know the amounts that you have both federally and provincially.  If you do not have your Notice of Assessment, you can verify the amount if you are registered for the My Account service on the Canada Revenue Agency website.  If you are not registered for My Account, you can contact Canada Revenue Agency at 1 800 959 8281.