You have to go fire, then frost, then 'sever magicka' and then 'elevate magicka', (the 'sever and elevate magicka' scrolls can be found in the chest behind Denel). And then the pillar will open leading you in...
Just did that quest this morning. Just make sure you save before attempting it, as if you do it wrong and waste the two scrolls you find behind Denel i can guarantee you will never find the same scrolls or spell of that nature again! Believe me i tried, Lol.....