heres everything you need to do.

first complete the 2 quests outside karazhan.

next you need to go to shattrath and talk to medivhs apprentice who will send to you shadow labyrinth.  if you choose to get the key it drops 100% of the time from the last boss in sethekk halls which is unlocked, or you can get a rogue whatever. 

Shadow lab is 4 bosses, and after you kill the last one you open the arcane container on the left side of the bosses room. dont open it unless everyone is there because it spawns a 70 elite elemental that you need to loot for the first key fragment which is the quest objective.

take it back to the apprentice and he sends you for the next second and third fragments. 

one of them is in steamvaults, all you have to do is go towards the first boss of the instance.  when you get close to the first boss you should notice a ramp going down in to a lake and the container is at the bottom, again a guardian will spawn who has the fragment

third fragment is in the arcatraz, the arcatraz key is a quest reward by obtaining the first and second halves of the key for that huge light thing named A dal next to the apprentice.  Even though the arcatraz door is a force field a rogue can still lock pick it if you want the easy way out.  take the ramp up in the first bosse's room and turn right into the corner of the room with the big demons upstairs for the last container to kill the guardian.

bring the fragments back and he will complete the reconstruction of your apprentices key, which turns out to be completely useless as is.  You are now required to go to caverns of time and enter the black morass but you cant enter black morass unless you first complete old hillsbrad in CoT.

Assuming your lvl 70, and you should be, durnholde is less than a challenge but after you finish it you will find black morass to be ridiculously difficult.

black morass or dark portal, is 18 waves of constant fighting.  each wave spawns a new portal which is held open by a rift keeper mini boss.  the portal will continue to spawn non elite dragons who charge against the dark portal and attack medivh.  waves 6, 12, and 18 are boss fights, and after killing a boss you will recive the only real breaks in combat to regenerate hp and mana.  as of the last patch you can no longer enter a completed instance and get attunement anymore so you must form a successful group after this you are attuned

How to get attuned to Karazhan

All of the above seems to be messing out on a step.

1) Talk to the mage outside of Karazhan. He gives you two quests, one to activate a crystal near a well and a pond in the underground cellars outside Karazhan, and one to collect ten essences from the ghosts in the cellars or just the ones outside Kara. Soloable, although the areas are quite packed and can hit hard with their magic.

2) The archmage sends you to visit a mage near Dalaran in the Alterac Mountatins. He can be found to the north of it.

3) The mage sends you to visit Khadgar near A'dal in Shattrah City.

4) Khadgar tasks you to go to the Shadow Labyrinth in Auchinedeon (which probably was spelled wrong just there) to get the first key fragment. It can be found next to Murmur, which is the last boss in the instance.

5) You're then tasked to go to the Steamvault to get the second fragment, which is in a lake just before the first boss there.

6) On to Alcatraz, the third key fragment is in the room with all the voidwalkers.

7) Complete CoT: Durnholde (Old Hillsbrad) to gain access to Black Morass (Dark Portal) in case you haven't done that yet.

8) Medivh himself will give you the last fragment inside the Black Morass.

9) Enjoy raiding.