the same has happened to me - inserted the three orbs, and...only one appears on the alter and door remains shut.
I've tired the 'knock' spell and nothing happens.
I do have mods installed - so unless i'm do something wrong with the knock spell ( i don't see how - it's point and click), i will have to wait for the next hot fix ( and pray) or uninstall all the mods (a bit of a nightmare as i've got Carried weight increased X900 and have a lot of sorting/selling to do beforehand) and try again
