As per this Reddit thread: http://www.reddit.com/r/SkyrimHelp/comments/n1yjj/under_saarthal_no_archmage_to_be_found_ps3/

I am experiencing this bug. I've almost completed Under Saarthal and the objective is "Talk to the arch-mage". I've returned to the College of Winterhold and the quest marker is pointing to within the radius of the Hall of Elements when I'm outside. When I go in, the quest marker tells me to go back outside through the doors I just came through. So I'm gathering that the arch-mage is on the roof of the tower, which I'm unable to get to via the staircase because it needs a key.

On Reddit they think it happens when you start the amulet quest before under sarthaal, which I did.

This is a showstopper for me - I can't do anything to complete this quest, and it's a pretty big one.

Any help would be appreciated, or a fix in the next patch would be great.

Thanks!

You can get to the roof through one of the other towers . I don't think he's up there though , he's usually in his quarters , which should be unlocked since you were told to report to him .
Try the upstairs roof doors , or try getting in through the arcanium via the upstairs or downstairs door .The arcanium connects to the arch mages quarters from inside . If all else fails , theres always the infamous " wait 30 days " trick .

Ahh , just read your link , maybe I misunderstood you . Can you get into his quarters ? The arrow still points up then ? That roof can only be accessed from inside his quarters once you are arch mage . Double check the chair by the enchanter , thats where I found him both times I did the mission , I've heard he is sometimes in the hall below , so make sure that's not the case .